Mordekaiser W passive

In Twisted Treeline, I was playing a game as Mordekaiser and had a duo lane partner, because jung Treeline is overrated. Anyways I come to notice that even though I was last hitting and everything, I would NOT get full exp for the kill UNLESS it was the Cannon minion. Don't know if this was suppose to be intentional but it just didn't seem right. "Rito, have mercy on me."