Credit for kills

Does anyone know how damage is applied to units to determine which unit gets credit for killing the enemy?

In my latest game, I had my stacks invade an opponent, it was early game so my stacks were mostly infantry, AA, AT, artillery and rocket artillery. My opponent was sending tactical bombers against me. I was giddy since I was going through hills and I was knocking out the bombers easily but I was surprised the regulars infantry were getting credit for the kills instead of the AA.