I may be following something completely goofy, but I recall reading about how the US designed the AR15/5.56 rounds to be primarily for disabling, whereas the global AK47/7.62 rounds were designed to be fatal. Idea being that if you wound one enemy, you can effectively remove two or more as the uninjured will try to tend to the injured; whereas killing someone outright only removes one. The AR15/5.56 is weak enough it's illegal for hunting deer in Virginia, so it seems possible.
No idea if that's a true story (or how to validate it), but if so, I wonder if Wisby was similar; thin the ranks faster by trying to injure people without being outright fatal?
Or possibly that leg armor was less prevalent and just an easier target, when you're talking about partially armored opponents a "lower value" but unarmored target is probably better than a "faster kill/value" target with armor?