It's possible to use the blockchain in a provably fair manner - see pevpot.com for an example. They use a future block hash as the input to a calculation that takes over an hour to come up with the winning ticket. So even if the miner is trying to cheat (perhaps by withholding a block if it makes him lose) he won't know whether his block makes him lose for an hour, by which time it will have been orphaned by some other miner.

I don't know what site OP is talking about. It is possible that it's a site like yours which claims to be provably fair but isn't. Maybe it even *is* your site - I don't know.