Maybe because WPG is a serious cup contender with boatloads of talent and that Buffalo is a completely **** team where the linemates Risto plays with are mostly AHLers. The reason his pp numbers look good is because thats one of the only times he's on the ice with other good players.
Ristos on the ice with everyone, that's the issue. Hes kn the ice with Eichel and hes on the ice with Girgensons/Larsson. Hes always on the ice, he plays comparable minutes to Doughty and Karlsson, and has no help. When Sabres have a PP is tu he only time hes on the ice with 4 other NHL caliber players.
I suspect his numbers come up as his ice time goes down, only time will tell, but I dont trade 4 cost controlled years for Troubas contract uncertainty. Hes the better player at this point in their careers, but he wants to get paid. It would be a bad decision to have Eichel at $10 million, and Trouba at @ $8 million if Dahlin becomes what hes expected to be and starts asking to be the highest paid player on the team in 2 or 3 years.