I think the difference is how much they push their luck. The likes of Loeb, Ogier or Rovanpera make fewer mistakes and when they do and get lucky it's quite obvious and seems unfair. The others make more mistakes, so the chance that they will find a big rock increases and eventually it happens.
Then there is also the theory that some supernatural rally luck moves from Loeb to Ogier to Rovanpera. We can obviously see the first two are not lucky anymore this year :P