@jcvjcvjcvjcv Well all units overkill their targets.
But that TMD overkills, while SMD does not makes no sense and from what i understand it's a stat of the unit being hit. So the missile has a stat saying how much anti-missiles should be fired at it at most, assuming each of those does 1 damage.
Since there is more than one AA weapon in the game, the air unit won't know how much damage incoming shots would do (and if they even hit), so you can't use the same mechanic on aircraft. But i think differentiating between units and uncontrollable missiles is fine.
Also, about the part with the shots not hitting:
Is there TMD that misses? And if so, how does it affect this? Will it shoot one anti-missile and if it misses your out of luck?