A revamp I think about late last year of ninjas, mercenaries, and necromancy was responsible for addressing many of the weaknesses with those sets.
Necromancy was always a serviceable set, but really shines with the attention to soul extraction, and the individualized pet powers, such as damage aura for grave knights.
Mercenaries and ninjas benefited greatly from the attention to pet Powers, the upgrades, and some passives. Serum in particular from mercenaries went from one of the worst powers in the game to a very potent must have pick.
Hence the reordering of primaries in the tier list.