Vehicle performance across different simulators/games

yesterday i did a quick comparison in the Mazda 787B at Spa-Francorchamps on the xbox one using a G920 racing wheel with driving force pedals and H-pattern gear selector.
On forza motorsport 6 i can do about a 2 minute and 6 or 7 second lap time
on assetto corsa i can do about a 2 minute 26 or 7 second lap time.

i could not work out why there was this twenty second discrepancy between games.