I don't know if the new game implemented it as well. But I think the old one had a mechanic like this:
The first time you fight against a certain mob type, you get additional AP for all of these mobs in the fight.
So if you encounter Orcs the first time and you kill 5 of them, you get 5 times the bonus AP.
If you encounter one orc you get the bonus AP for that one, and if you encounter 4 more, you would only get the base AP for these.
But as I said - no idea if its implemented in the new game.