Armour has a random factor of 50% to 100% protection. Blame the creators of warband for this.
Also, there is a sweet spot for dealing damage. At a tap it will be 100%, but hold for 0.6 seconds before release and it will peak at some ridiculous number like 150% damage. Hold it too long and it runs back down to 120%. This is not just for melee, but also range iirc.