I'll give you my take on bitcoinbetting.website's provably fair system, since TrevorXavier isn't going to comment. bitcoinbetting.website is for the most part, provably fair. For bitcoinbetting.website to cheat, they would need to own a large % of the network hashrate, and if a winning bet is in the block they just found, they could withhold the block and let another miner get a different block, with possibly a different last digit. This would be hard to do though, as the hashing power needed would cost a lot of BTC, and the payout has to be over 25 BTC for it to be worth it to withhold the block. I doubt they have that much money to get a large share of the network, and the bets are still pretty small so I am 99.99% sure they don't cheat. They can, but it is extremely unlikely.

TL:DR : They can cheat, but they probably won't unless they own a large amount of the network's hashing power and the payout for a bet is over 25 BTC, and is included in a block that they mine.