It's a passive "always active" ability in the weapons. They ignore defense and use a completely different equation to deal damage.

Because the game wants them to? After all, with increasing difficulty, there comes the burden of increasing damage. Are you looking for some kind of proof? I mean, they do that damage because it wants them to. Games don't follow logic. If they did, I'm sure we'd all be discussing perfect numbers or graph theory instead of suspending disbelief.