Skills and Super Ability of the Orcs.

I have a question regarding the Barbarian's main skill – Blood Fury. I can't seem to get Absolute Blood Fury to drop; maybe I'm doing something wrong? But getting Adept Blood Fury isn't a problem, so why don't I get the next skill in the sequence afterward? Or is it only possible to obtain it by equipping the Amulet of Mastery? I would be grateful if someone could explain what the issue is.