Gambling is all about random number generation, and provably fair scheme provides guarantees that casino doesn't cheat, but we also know that humans are horrible random number generators - we are very easily predictable.
There are many online demonstrations available, I'll just drop one here:
http://www.cs.stir.ac.uk/~kms/schools/rps/index.phpAfter a small number of games, computer becomes quite good at consistently beating you at the game.
So, theoretically a casino can observe how you place your bets, then predict your next bets and make their own bets (they always go first) accordingly. The flaw here is if this algorithm is implemented too naively, it can be abused by players by making big amounts of predictable low-value bets, and then making a big opposite bet.
We really are horrible random number generators but I don't think that gambler site owner won't implement algorithm too naively where what you mentioned is true that it can be abused by players who knows that it has a flaw that you can take advantages. I think they will use algorithm but I think it's more aggressive in giving randomly generated number that make the game base on pure luck.