amok: I have feeling that after a certain amount of time we will see a silent change of publisher name her on GoG. It seems unlikely that whoever have the SirTech IP's (or the Wizardry IP's) are going to continue splitting them between two different publishers for the global market.
You're reading too much into it.
I can give you two immediate counter examples- if what you say is accurate then BitComposter would publish all versions of Expeditions: Conquistador. In fact, they publish only the steam version (specifically done to subvert greenlight) and here, gamersgate and everywhere else the game is self published. There's also the BGEE situation, go to Beamdog and the BGEE is self published, yet Atari released the game onto the other portals without even telling Overhaul about it in advance. While split publishers are unusual and can lead to complications like the Drakensang situation they are not unheard of, and publishing does not mean any ownership rights, just a licensing deal.