I think it is something to do with multiple numbers of reasons all stated above oozing and mixing up to create a strange effect. Particularly something to do with the damage models mixing it up with gunnery models.
First, I don't think "I know I hit something" is very objective. You send out a birst, bits and parcels come off... but was it all your shots landing and ripping off some vital chunks? Or is it a few rounds cropping away dust and blowing useless holes? Everytime when somebody complains about "1 ping fatality" in AH, he's met with the classical "that 1 ping could be 1 round hitting, but it also could be 10 rounds hitting. U don't know for sure" answer(which seems reasonable to me, anyway..). I don't see why this sort of logic shouldn't apply to IL-2.
Second, maybe the damage model is drastically different from what we've seen in AH. In AH, something gets immediately destroyed. All-or-nothing. No damage or total destruction. I can't say that MS CFS has modelled damages more properly than AH(since, yeah, I know jacksh**.. I've never in WWII

), but in CFS, damage models seem to make a bit more sense to me.
'Fatal' damages somehow never look fatal. Things don't just rip out and show as easily as AH in CFS. Many times I shoot at a plane, and it seems to fly on and I chase him and dump everything just to blow him up. It wasn't until later that I found out that the target plane, looking pretty healthy to me, was actually a flying brick. I chased a plane I shot for a moment to see what happens.. the damaged plane, though it seems it was still flying, would just go plain straight, can't move around much, then after some time will lose alt and plummet to the ground. Or in other cases, the plane I shot flies fine straight.. until I chase him, he tries to move away, then can't recover from his roll and drops to the ground. Or sometimes, the plane just seems to be fine.
AH is crisp and clear. It's easy to find out if u've shot him and done damage. I don't think it's quite this clear in IL-2.
I wonder if there is a hit percentage data of the single sortie one flies in IL-2. This would help to clear up if IL-2's gunnery model is a 'screwed' one, or a 'realistic' one. Until we get to judge upon that sort of data.. I don't think we can do much more than just oggle around and keep guessing
