I've been thinking about this recently myself. I think one factor is you have less tables toward the end of your session so you are probably more focused and playing your A game
It could be that, but i generally play 3-4 tables most of the time at the mo as im just getting into playing a bit more used to do 6 easy.
I think it could be a case of concentrating on the one table and thinking this is my last game so put the extra effort maybe??
I would love to start a session on a high so i can just continue to crush