Of course you don't get any bonus in the old engine because you don't need it. It's a physics based model. It you study the trajectory characteristics of an arrow in the old engine, you see overshoots and undershoots. The deeper the formation of the target unit, the more hits you get. Since archers work best in wide, shallow formations, that's how they are usually used, and if you enfilade such a formation, you will get more kills. I've tested this and I'll look for my test results, but I don't know if I can find them.
And EXACTLY the same is STILL true in M2TW. Based on what I've seen it appears to use a calculated number of hits, but the animations that achieve this are set to play out so that the offset distance (i.e. the distance those that should miss aim off target), is the same at all ranges as it would be at 45 degree. Thus it is small enough that at shorter ranges, (or in enfilade fire situations), the arrows will still hit the target unit. The animation override also ensure that any arrows that DO hit will trigger a kill check.

Now of course I don't know for sure that the above is the case, but if I set out to create an engine that duplicated the effects I've observed, that is how i would do it, and it's the end effect that matters really isn't it?

it also explains why shots at targets out of range hit, the fixed accuracy kicks in and forces a hit.

One interesting point however is that no matter how much you increase range and accuracy, if the arrow doesn't have enough velocity to reach that range then nothing will make it actually reach that range (the unit won't even open fire), so clearly, animations are being taken into account both before and after the firing.

My best guess is it's the STW engine with a fixed accuracy added on so as to remove accuracy variables relating to the movement speed of the enemy unit. this naturally makes it a lot easier to balance and a Little easier to learn the game.