I don't buy, even for a second, that the PL instructs ref's help United or Liverpool.
However, like in any job, you can be influenced by wanting to please your boss.
So if you hear your boss making statements about needing certain clubs to be successful in the league, then human nature can come into play.
Just as negative press can create a negative human bias towards cheating Arab bastar.., sorry certain clubs who don't have American owners.
Another way to influence (and was reported by ex referee's), is to give them lower class and less well paying matches, after perceived decisions not favouring certain clubs. Although i don't think that is an issue currently.