If a reel had 20 symbols, then the chance that a symbol would appear on the paying is 1 in 20. In other words any symbol had an equal chance of showing on the payline. In the new computer controlled slot machines this is not the case. There may be 20 physical stops on a reel but each stop does not have the same probability of showing on the payline. The computer uses a mapping technique to convert the large number generated by the random number generator into a smaller number or integer that is mapped to the stops.
Casinos can set slot machines to pay back more during the week and less on the weekend.
Casinos do not fiddle around with slot machines to change the pay out percentages. In Atlantic City, casino aren't allowed to change a computer chip in a slot machine unless they first notify the gaming regulators and then a specialist from the Division of Gaming Enforcement must be present when the chip is changed. Casino managers usually change a payback on a slot machine when they order a new machine. In fact slot manufacturers provide a spec book that contains all the available hold percentages for different models and denomination of machines. Slot machines pay back more when you use a slot card.