AI enemies hit between them.
This happened to me many times.
For instance, there are 2 enemies. They come to you, so you have one enemy in front of you and one behind him.
So the enemy that is farther hits and makes damage to his ally, that does not retaliate.
I use this trick in crowded areas to help me finish tough guys.
Maybe the enemies could in-fight between them like in Doom or maybe the AI could be improved so the enemy in the background circles his ally and comes to kill you.
(RM-3254 from redmine: created on 2016-03-14 by Ajira the Khajit, , closed on 2017-02-12 by R. D.)