I don't know exactly how the damage calculations are done, but it is not a straight weapon damage minus armour rating equals damage taken. I'd have to let the same opponent hit me with different class armour to get a better idea of how it works; just playing through the game the opponents gradually get stronger as you are upgrading different pieces of armour, so I certainly noticed a difference a few times, but a direct correlation between increases in armour and decreases in damage (at least partly because I rarely paid attention to the individual damage numbers, unless I was specifically checking something).
