Costa over Dybala this season has to be a case of just remembering fresh memories of the last month instead of the whole campaign. 20 goals are huge. That difference buries any attacking player.
In the build up, Dybala had more passes, higher passing accuracy, more key passes, better shot accuracy, etc. On top of of course 20 more goals.
No comparison between Dybala's and Costa's contributions to our season. Its a double standard. Expectations that were exceeded have no bearing on who the player of the season is. If your expectations were lower for Costa then just be pleasantly surprised, that doesn't mean anything about their performances.
Had Costa played the whole season like he played the last month then its a different story.