My narrative explanation is thus:
The areas of your body that are most vulnerable are also the most well defended. You're very protective of your face and your vital organs.
A normal shot goes for a limb, because legs and arms are big enough to serve as targets but aren't as heavily armored as the torso or head. The sharpshooter is either trying to hit the upper chest armor perfectly dead on (so it pierces the armor instead of being deflected away) or trying to find a gap in a visor or neck guard. With shots like that there's no such thing as a partial hit; you either shoot him right in the face or your shot is deflected by the armor. It's a binary result that way.
Even for unarmored folks you're a lot of more conscious of ducking when it's your head that's getting shot at than your arm.
Anyway, that's good enough for me.