They are truly supposed to be random. But it is said that some casinos program the machines with less probability of jackpots. So what people like you and me do is play at a casino which guarantees 100% fair game play. Most of the online casinos guarantee that.

Interesting question- slot machines are actually based on a degree of randomness in each individual play, but the payouts when those random events produce a winning combination are controlled to ensure that only a certain percentage of money played is paid back. Typically in the high 90% range.