Basically, due to unavoidable internet lag and the speeds at which WWII aircraft move, your relative positions can be dozens of yards apart on each Front End. So, Guy A can see a guy dive behind him, 100 yard back on his FE, while Guy B sees himself fly through Guy A's airplane on Guy B's FE. Now, Guy B rammed Guy A, but Guy A had no way of telling that he was going to get rammed and thus had no opportunity to avoid the damage, thus only Guy A, on who's FE the collision happened and thus had potential access to the data needed to avoid the collision, takes damage.
Sometimes people respond by saying "Well, then neither should take damage.", but that distorts the air-to-air combat tactics by making it a good idea to fly right through the enemy aircraft, firing from insanely close ranges. I won't miss that B-17 I am diving on at 500mph when I fly through it, whereas the B-17 gunner sees me dive 50 yards behind him, making his target much harder.
Basic choices are:
Both take damage. This leads to ramming being a viable tactic and being very hard to avoid as the player on who's FE the collision does not occur is not able to see that the collision is about to happen.
Only the player on who's FE the collision occurred takes damage. What we have in AH, intentional ramming is very hard and the player who could potentially have had access to the data to avoid it takes the damage.
Neither take damage. Leads to people flying through other aircraft, guns blazing, lack of concern about collisions being a pretty big distortion of air-to-air tactics.
Only the player on who's FE the collision did not occur takes damage. Rammer's wet dream as only the player could not have potentially had access to the data about the collision takes damage, rammer gets to ram over and over and over. Insane and obviously not a good idea.