Oh, well, if the calculation in that link is correct, then I’m pretty sure there should have been no hit point damage at all. If 20 damage was done to armour, that means that the base damage could’ve only been in the low 30s, and that the % ignoring armour could’ve only been a tad above ten, and that a tad above ten should’ve been deducted because of remaining armour.
However, if 10% of the remaining armour is deducted from the value of damage that “ignores” armour then armour is hardly being ignored. I wish the in-game info on this was clearer.