Game Design: Casinos design their games to ensure that the odds of winning are always in their favor. For example, in roulette, the addition of a single green zero (or two green zeros in American roulette) gives the casino an edge over players.
While the results in slots are random and determined by a random number generator (RNG) within the game, there are some fixed parameters that determine the chance of winning. For example, the number of symbols for each type is fixed. More matching symbols on the reels can result in more frequent wins.