I think that it was more like Webb refereed more "important" United matches than, say, Manchester City matches. It was in those matches where controversial calls tended to go in United's favor, leading everyone to accuse Webb of favoring United.

And Liverpool with the most losses.

Well it is not unjustified . He did made so many , very many controversial calls in United matches .

This chart is useless, it's extremely not on point and proves nothing other than that the top 4 teams in England... are actually the top 4 in England...
Just my 2 cents..

He said himself he's United's fan thats why people make joke about it. And because of that, he knows people would pay more attention to him when when he ref MU's games or games that affect MU's result.

He made mistake sometimes, so do all the others. Great ref IMO.