I've just tried calculating the damage to the player (and/or other actors) by backtracking from the damage to their armour. Strangely, condition loss/GetGS fDamageToArmorPercentage returns a value that's always exactly 9x lower than the actual health loss. In a weird but possibly irrelevant coincidence, fDamageToArmorPercentage was 9 by default in Oblivion.

Anyone bumped into this before? Can anyone spot a gamesetting I've missed?

E: Oh no, it turns out it was only 9 against two people's weapons. Against others it flips about randomly. This is kind of aggravating.

Double E: Solved. Turns out damage to armour is proportional to the player's current AR (higher ARs = more damage dealt to the armour).