This seems to only happen with melee attacks, casters and bows calculate AP properly. Fortunately I'm running a caster-heavy team right now so it's not that big a deal...I really want to try a melee-heavy approach at some point but this is really putting me off.
Has anyone else experienced this? Any explanation?
I think I've had this happen with a crossbow as well. I shot once with a crossbow for 7 AP, and for the next shot the game reported I needed 8 AP. If I remember correctly.
As for melee... I'm not sure, but it seems to me that characters sometimes move a few pixels after being hit. Sometimes a character is next to fire/poison for example, gets hit and suddenly bursts into flames. But I don't know, maybe they already were in the fire in the first place.