Before we all get up into our re-enactor/armchair historian mode... the SR penalty rule is only there to provide some sort of reasonable consequence for the wearing of armor, and to make the consequence higher for better-protective armor, in some sort of linear and predictable fashion. That's all.
It isn't supposed to be a perfect simulation; just to provide a drawback of some kind to balance out the benefit of armor. "Tires you out" isn't an effective consequence for a game, because many fights in MRQII don't last as long as battles did historically. % to skill penalty didn't work well in MRQI (as you know if you played that version). So they had to try something else.
I think the SR penalty, while not perfect, works quite well. The penalty is based solely on total APs. It has nothing to do with the fluff in the description. You could spend forever debating this type of armor vs that type, weight materials flexibility butted riveted eastern western oh and what about the byzantines nar nar nar. But since it's actually a game balance issue, none of that matters at all, really.
