That is complete and utter nonsense. The only set I don't remember getting on my guardian so far is the Fallen Leviathan set. It's all just a matter of luck. The only constraints on which set drops where are regional - some sets drop from every boss, others only from bosses in a certain region - Cimmeria, Aquilonia, Khitai, Tortage, Stygia (the latter includes Turan and the Dragon's Spine).