I just won 5:0 and my GK got rated 6. I don't think he made that many bad tackles
And as for the win bonus:
What i could imagine is that if you play against a lower rated team (quality wise) and you have winbonus set also for normal league matches (always active) and the lower rated team only uses win bonus clever (i.e. against stronger teams or important matchups) it turns into a bad beat...this is THE ONLY REASON i can imagine that this kind of result could be possible.