I'd say it has to do with the fine line between peak performance and a little less, and in events where technique and timing is critical. Take two top exponents who are really close to each other in peak potential but only need to dip fractionally from the peak parameters required to produce their best for one to out-perform the other on a given day. For instance, Tiger Woods wins an awful lot, but not every time. The PGA advertisement is really accurate: "These guys are good!". The bottom line is that aside from rare exceptions such Sergei Bubka, Tiger Woods, Phil Taylor even, being good is one thing, but being unbelievably good is another. There's seems to be something intangible about the way champions produce their best, but apparantly even they cannot always control the result that a slight dip in performance produces.It's not really a very scientific answer but as a golfer who can play a bit after once being a hacker, I appreciate the skill and mental control someone like Woods possesses. The distinction between suceeding and performing are entirely linked in his case. It's unbelievable the number of clutch performances he produces, but there's always the chance someone else will experience and produce a perfect physical performance and beat him.

I think that it has to do with minor adjustments to mechanics. Just a few inches can change a person's performance for good or worse, so two evenly matched people will vary slightly in their style everyday and some days be a little better than others.That being said, pros are very good at keeping their mechanics the same all the time so they will be fairly consistent, but nobody is perfect.

For middle distance or distance running, shown in the picture, the answer is that the event is a tactical contest. No runner goes out do a 'personal best' every race, the intention is simply to beat the other runners. If you are up against a runner who is inherently faster than you, it is eminently possible to beat him using tactics including varying the pace during the race. Anything from 1500m to the marathon is actually a multi-player tactical game, and the best prepared runner will win, not necessarily the quickest runner. One coach, watching a race with 15 men which started very slowly, made the comment "one smart man and 14 stupid men"!Needless to say the same smart tactic is not likely to work again against the same opponents.

I would it say it depends on a variety of factors. What the person ate that day, physical condition(e.g more tired than usual), emotional status are all factors that can affect the performance of an athlete...

As an athlete and sport science graduate I will use the broad subject ares to explain, Biomechanics- all of the disciplines mentioned, sprinting, swimming etc require the athlete to have advanced technique to be at the top of their game. environmental factors, luck and psychology will all affect how consistent the performer is. Any one of these variables could affect the consistency and at the highest level (summed up nicely by the lucozade avert) winning can be a matter of centimeters.leading nicely on...Psychology- one day you may wake up and your head not be in the right place so to speak, or conversely right on the money. Arousal levels, or IZOF (individual zone of optimal functioning) will be affected by opposition, weather, confidence etc If the athlete isn't in their IZOF then performance will be affected negatively -for example the javlin thrower who sees their opponent throw a massive PB and lead the heat may internalise that as pressure leading to increase arousal this has been found to cause performance decrements due to loss of focus on correct techninque. Pre-performance sprinters try and pysch each other out to give them an edge apparently the year that Linford Chritie won the gold he looked over at one of the other competitors mouthing to their trainer about the shape he (Christie) was in and from the moment he knew he had them beaten.Physiology- hydration status, energy intake, temperature, altitude, training volume, and injury are just some of the variables which an athlete has to manage. being human some times preparations don't go as planned and so they won't be at their peak potential. the fact that everyone is different means that different people will react very slightly to the very different environmental conditions they have to deal with. for example the athletes had to acclimatise to Beijing and so some did better, faster, more efficiently than others, some times that also comes down to funding.There are many reason why not one individual will totally dominate when two closely matched individuals compete I hope i have managed to give a well rounded (if general and brief) overview.