I think in support of this theory is the fact that the level 80 XP bars are now showing how much you have, out of how much it takes to get to 81. I never saw that before the patch. So maybe it is actually calculating and storing that rested.
If not, stopping at 79.9 means he's only like 1 quest turn-in away from 80 and being where everyone else is. Good risk versus reward.