So, in some games, the damage a player does is modified depending on whether or not you're attacking a player or an NPC. So basically it works:

Ability does X damage vs NPCS
Ability does X modified damage against players

It seems like an efficient way to solve PvP balancing issues and allow for greater flexibility for developers, though admittedly I don't know how hard it would be to implement. But in the long run it may be worth the effort to avoid the constant headache of class balancing for two very different game types.