I think they have problems with certain players. Rooney, for example, would get a red for pretty much anything for quite some time, although he's been lucky to avoid them for a while now. Meanwhile, Vidic gets away with murder. Likewise, you foul Ronaldo and you'll get a yellow (sooner or later), meanwhile Evra gets the nastiest ankle-targeted tackles on a constant basis but I've only ever seen one person carded for them. I'm shocked Evra's not been injured, personally, but eh. But yeah, I reckon the FA/referees just don't like certain individuals.