Source? And why would they decelerate? Extrapolating from your last sentence – fatigue? Would be interesting to compare acceleration at 80m in a 100m sprint vs. a 200m sprint.
https://en.wikipedia.org/wiki/Bioenergetic_systems#ATP-CP:_T...
Yes. They train to exert the maximum amount of effort in order to optimise their time. If they're at maximum speed at the finish line it does them no good, they likely could have expended more energy to be quicker in the middle of the run to cut their time.