Casinos make a profit by building an advantage right into the rules of the game. That's reflected in the payout percentage and it applies to all online casino games, not just slot machines. But, like we said before, that's just an average. In a single session there's always the potential for a big jackpot or, on the other hand, a cold streak of rotten luck. That's one of the reasons people love slots. They give you the opportunity to blow the payout percentage out of the water with one big win.