Originally Posted by Bobo
I believe that when you attack that unit, your arrows tend to target the center, so men positioned at extremes have a very low probability of getting hit. A similar thins happens when you attack a running/routing unit with cavalry, where one or two men are running far behind. Your cavalry will then run in between those subgroups hitting nobody.
Incidentally, have you then also noticed that when they shoot arrows straight up (to get to your men behind walls), they actually aim normally, the arrow actually appears normally, but then the arrow all of a sudden goes straight up? Have you also noticed that sometimes the arrow appears way after the shooting animation (I've seen that with Aztecs).