[andy800]

Roulette is perhaps an easier example. A typical American wheel has 38 positions: 1-36, 0 and 00. If you bet on a single number and hit, a 1/38 chance, you don't win 37x your bet, you only win 35x. If you bet on Red, or Odd, those pay 1-1 but the odds of winning are less than 50%, there are 18 winning results and 20 losing results. The payouts are always lower than the true probability of that specific outcome. If two people play opposite colors, one Red and one Black, the casino will break even on numbers but collect both wagers when 0 or 00 wins.

Another example is sports betting with a point spread. You bet $11 to win $10, on supposedly an 50-50 wager (the purpose of the point spread).

Slot machines work off the same principles. It's ok for the outcomes to be random, because the payouts are always less than the true probabilities.