Well it is not a plusminus award. A quick look at his on-ice saves percentage of .945 tells a lot about where those pluses came from. Besides him scoring 43 goals with over 23 shooting percentage.
If there was an award for him to win, it would be the Unsustainable trophy.
All due respect, Wild Bill has played awesome, his team has played awesome and are deserved favorites to win the final series. But plusminus is a stat that comes from things not under one player`s control, especially in the cases of these huge deviations like his +49 is.