Mob block has to function slightly different than player block. I don't know entirely how, but that is the only thing it can be. Mobs can also block ranged attacks, for what it's worth.
I've tested with a warrior and shield block, and while players can block ranged as well, it does not work as it should. Namely, with over 100% block, you don't actually block the shot. 100% of the time.
Screenshots (pardon the old messy UI):
