Out of all the months FXCM has been having the trading competition, I think a top 5 winner has only made it back into the top 5 once. So it's not as though the winner is doing this consistently month after month.
You're looking at a case of survivorship bias since there will always be people with outsized returns given a large enough sample size. As others have said, I'm guessing that the winners of these competitions are using a highly leveraged strategy (FXCM offers 400:1 so these returns are not impossible).