Racers consistently obsess over their position, trying to find the "perfect" position. But this is virtually impossible - hitting the "perfect" fit is like trying to hit a moving target. As your body adapts to riding, you'll find that your position can change to accentuate the adaptations your body has made.
An extreme real-time example of "fluid optimal position" is Eddy Merckx - due to a devastating early career crash that killed his derny driver, his hip bothered him for the rest of his life. His intra-race seat height adjustments are documented in "La Course En Tete" during the World Championships. A lessor example of changing optimal positions would be the recommendation I read somewhere which stated that during a stage race, after the first really long day, drop your saddle by a couple millimeters - this will relieve some of the stress on tightening muscles. It works too, although I apply it to my training camps rather than stage racing.
Your position will vary throughout your riding career in a less extreme and less fluid fashion. For example, when you first start riding, you tend to be a more upright, perhaps what one could call the "Entry Level" position. Non-cyclists are usually optimized for walking - so your glutes are not well developed, your hamstrings and your lower back isn't happy being bent over, your quads are not that strong, and your triceps, neck, and upper shoulders aren't happy supporting your head as cycling requires. The most foreign measurement for a new cyclist will be the seat height - it usually feels much lower than a non-cyclist feels is right. This is because the human body is optimized for walking and that involves straight legs. You don't walk around crouched like an ape, do you?
The Entry Level position holds true until you develop some of the aforementioned muscles - the glutes, hamstrings and lower back become stronger, your quads are usable at less-than-fully extended positions, and your upper body muscles can support your head correctly. If you find yourself hunching over more, bending your elbows more, then you're probably ready to move to the "Intermediate" position. The bars are a bit lower and farther away from the seat. Your seat may move back a bit. In such a position, you substantially increase your power by recruiting your glutes, hamstrings, and lower back - and when they develop in response, you can alter your position again.
Of course this adjust-and-adapt cycle will level out and at some point you'll find yourself in the "Optimal" range, with the only adjustments you make for riding preference (time trial, road race, crit, mountain bike), gear (cold weather gear is thicker and sits you higher off the saddle), any physical issues (stiff back, tight shoulder, etc.), and any personal preferences (long hilly ride versus super short/fast crits).
The Optimal range is relatively diverse for those of you used to measuring in 1 mm increments. I find my seat to pedal distance rarely changes but my saddle will move back and forth up to 2 cm. Move the saddle forward requires it to be raised (to maintain seat-pedal distance) so my seat post does move up and down about 1 cm. When my riding requirements rarely change (the length of the fall and winter for example), my position doesn't change. But when I'm trying to optimize for particular races, I definitely try and optimize my position about a month or two before hand. This means I am adjusting towards the short, fast summer events by the time January rolls around.
From personal observations, I would say that one's Optimal position is pretty set after about 3-4 years of consistent riding, i.e. relatively serious riding for 3 seasons for 3 or 4 years. This can be radically altered if there is a substantial change in riding requirements. For example, Davis Phinney transitioned from being a top a 100km TTT rider to a pro doing much longer races and eventually won the Coors Classic stage race. In the process he substantially lowered his seat and it appears he moved the cleat back on his shoe. Injury can alter position - probably the best known is Floyd Landis - his hip issues dictated his unusual position on the bike. Free thinking radical experimentation can lead to some interesting bikes - Steve Bauer comes to mind, with his chopper bike with chainstays so long it required the builder to use two sets of stays brazed together. His power position got him to the end of Paris Roubaix at the front but he had sacrificed speed for power with his position and couldn't sprint for the win. Minor position adjustments are relatively common - Bernard Hinault fit a 1 cm longer stem to his bike just before winning Tour of Lombardy in a spectacular solo break. When Greg Lemond joined Renault-Elf, his seat was raised something like 5 cm and his feet straightened out. His spectacular results show that this had no ill effect on his racing.
As I mentioned before, the one thing that rarely varies is the seat to pedal distance. Your pelvis may sit farther forward on the bike (for shorter, faster events) or a little more to the back (for longer events emphasizing power). This doesn't mean your seat post doesn't move, it simply means your seat-pedal distance remains pretty constant. For example, changing pedals may drastically alter your seat height relative to the bottom bracket and require a seat post movement simply to maintain your seat-pedal distance. Moving the seat forward will definitely require the seat post to be raised slightly.
If you haven't changed your position recently, it simply means your current position is set up for you as of the last time you changed your position. But if you have changed at all since you set your position (you ride more, you ride faster, or conversely you gained weight, you ride less) then it may not be ideal anymore.
Staying optimistic about fitness, as one gets fitter one can usually got to a more compact (or smaller) frame. As long as the top tube remains the same length, you will probably be able to drop your bars more relative to your seat. This will probably make you more aero and improve power without changing your seat-pedal position. If the frame is also longer, it will stretch your upper body out a bit. Big bike manufacturers usually have two sets of frames to satisfy the new and the advanced rider. Giant, for example, has the TCR for the more fit riders and the OCR for the newer riders. The TCR has a much shorter head tube and a slightly longer top tube - optimal for a fit rider but uncomfortable for someone trying out road riding. The OCR is the opposite - a more upright position for those less fit or simply new to the sport.
Position trends seem to be cyclical. You may end up emulating Jens Voigt's "new" position, a very forward, very high seat, coupled with a faraway, low bar. Of course this was also Alexi Grewal's Coors Light position over 10 years ago. The forward position allows you to severely flatten your upper body without crunching up your diaphragm. Works for them, it might work for you. All position ideas have their advantages and disadvantages.
A note on Jens Voight - his position looks very forward but it might be a function of the seat tube angle - a shallower angle will allow the seat tube to fair the rear wheel better and the designers may have chosen to have a bike that's more aero than one that "fits" better.
Higher/forward seat - more speed, easier to spin, necessary for short-quad folkds, instinctive position for high effort bits. Being "on the rivet" means you're going as hard as you can. It comes from the idea that in the old days, the riders would scoot forward on their riveted seats until they were "sitting" on the front rivet. Hence "on the rivet". This position tends not to be ideal for ultimate power - say on climbs.
Lower/rearward seat - much more power especially at low rpms, more recruitment of glutes, lower back, hamstrings. Works well with cleats further back on shoes. Necessary for those with long quads. Allows for more movement on bike so it's easier to, say, bunny hop or skip the rear wheel around.
Long reach to bar - more aero, more forward position when out of saddle (better for climbing and probably for sprinting). Less able to react to "exogenous" bike input - in other words, when someone slams your bike, it's harder to stay upright. Less weight on seat. More weight on hands (requires gloves or better gloves).
Shorter reach to bar - more comfortable, improved bike handling. Less stress on upper back, triceps, and neck. More weight on rear wheel so more stress and wear on rear wheel and tire. More weight on seat. Less weight on hands.
Note - you can combine one seat theory with one bar theory - so you can have a more forward seat with a shorter reach to the bar.
Remember that although you may alter your position on purpose, any unintentional position changes will affect you significantly. The first link (to the Discovery rider getting his seat position checked) is probably not the rider saying "Hey, I read a blog on changing seat positions and I want to try my seat a little higher." It's probably "My seat should be like so and it isn't right/level/etc." Racers are meticulous when it comes to seat position but it does not mean they can't ever change it.