Well I believe it's still a function of attack-defense but the "hidden lethality factor" is probably not so hidden but a result of the animations.
In M2TW, we have a 3d collision model between soldiers which means for example that it's very hard for archers with their small daggers to hit mounted units, and at the same time makes a lot of 2-handed units fare worse than you'd expect against other infantry because they are too close to them to hit them effectively with their large weapons sometimes.
This also applies to shields where the defense value is in fact only applied if the defender actually plays the block animation.
So all in all, it's very complicated and devilish to balance.