Defense is not a silent variable. It is working..... I told ya that it is not meant for bosses only, on players it works normally.
And that makes sense as well, as bosses are mythical monsters and some are mythical gods as well, so they have their natural defenses rather than equip defense..... And natural attacks....
So the bosses have resistance to your eqp def.
Thank god they don't have resistance to your eqp str, or they would have been immortal, lol