Because it isn't.
Previously, resistance would deduct from your attacker's %dmg until it reached 0. Now it's calculated as a final resistance. However, Final Resistance is not visible on your character's sheet when right-clicked, only your converted resistances. Therefore, for clarity's sake, buffs that used to add final resistance now add the equivalent amount of real resistance so you can see what the actual resistances are when you're attacking somebody as opposed to the value not being correct.