There are guys who are fast, then all of a sudden they ride terribly. Sometimes it's as simple as their saddle slipped down on them just 1cm or so.
Little things make a big difference. This is why people say that track racers are really particular about their bikes.
I once used shoes of two different brands (Bont and Shimano). I'd have to move my saddle up/down a few mm depending on which shoes I wore. Otherwise things were just off for me. Now I use shoes of the same brand to keep the stack height the same.