When you are on a Battle Arena area (like RBF and the Duel Areas like the one above Heidel), it ignores equipment and just follows the AP defined on PlayerDuelAreaAttack on the battleoption.properties file.
You can test this by literally removing all armor and damage will still be the same, depending on the AP set on that line.
PvP outside of these areas liked Forced-PvP, however, follow the equipment gear.