Some sites are most likely rigged and/or have different way of sharing cards.
But really, the players are different, some are worse, some are better, some happen to be drunk, some on drugs.
If you let things like this **** up your head, even if those weren't true, it will effect your game and can be seen from results. It's not good to be thinking that: "Hey, these players all are solid and there's no money here.", you should be thinking/trying to find out what mistakes they do and then they take their money because there's always a way to get their money if they play.
Of course the results might be just different because of the skill level, but what I'm trying to say is that you don't want to make it any worse just by thinking unnecessary things.