Fangraphs uses UZR, B-Ref uses DRS. UZR has him at +17 and DRS has him at +5, so there is your difference. I know the DRS methodology has been updated regularly. I am not sure when UZR was last updated. Statcast has him at +20.
Just because UZR hasn't been updated though doesn't means it's wrong. When I was paying more attention to fielding stats, there wasn't a good way to account for shifts though, so that might be the reason for the discrepancy. On the other hand, +17 or +20 seems awfully high for Schoop given his past record.