Automatic defensive DPS

I have noticed just recently that a lot of my defenses waste ammo (cannons, archers, etc.) when defending my base and precious resources.
My idea would be to have an automatic DPS calculator when a defense building targets an enemy unit.
For instance, let's say that an enemy drops 5 level 6 archers (44HP) right inside the radius of 1 level 10 archer tower (65DPS). Since the archer towers shoot 2 arrows per second then the damage PER ARROW would be 32.5. SO, for an archer unit that steps inside the radius of the archer tower, the archer tower ends up shooting 3-4 arrows at an archer unit that only requires 2 arrows to kill since the archer unit is standing at the furthest distance away from the archer tower but still within its radius. Of course the archer tower will target the first and closest unit that comes within its range (aggro) BUT here lies the problem; the archer tower wastes 1-2 arrows on an enemy archer unit that has already been killed because of the distance the arrows have to travel and the speed that the archer tower shoots.
Why can't there be an internal recognition of how much ammo is required to kill a particular unit so that the defensive building can move onto the next enemy unit without wasting time/ammo.