Near as I can tell, it doesn't matter where they actually hit you, just that they hit you in the first place. So it's not calculating the likelihood of which body part actually got injured and what armor you have there to determine your damage taken. It's calculating a couple of other factors though, like your general armor rating, the resistances granted by your respective armor, any resistances you may have from other sources, and sometimes factors like how much total health you have at the time (you get staggered easier with less health, being hit while staggered tends to hurt more).

Overall it means you want to avoid getting hit in the first place, and when you can't avoid the hit you could use shields to either block additional damage or parry the attack but that when you are hit, it doesn't matter where on your body they manage to hit you so much as how hard they hit you and what damage type they deal when they connect.