If anything, Poison would be the one getting a buff. Being Super Effective to Water or something.
I mean, they have changed the type chart before, with the transition to GSC (Poison being nerfed and the addition to Steel and Dark). Maybe they thought that Ghost was too powerful or Bug was too weak? Though, this does make stuff like VoltTurn much more difficult to stop (Golurk being a Pokemon that can't resist the combo anymore).