Poor teams get more decisions going against them and good teams - for them. It's both the subjective judgement of referees, as of course it's easier to award a 50-50 penalty against Crystal Palace than against Man U at old Trafford. But the other aspect is that better teams generally create more chances, have more of the ball and so to say more opportunities to get into a position in which a game-changing decision has to be made. If Palace get a player of theirs tackled in the opposition box around 2 or 3 times per game, Man U have that at least twice as many times, thus more of a chance to get a decision go their way, especially combined with the subjective factor.