The random number generator is constantly calculating random numbers. These numbers roughly correspond to the symbols on the reels. They can program these numbers to come up a certain percentage of the time, and that percentage multiplied by the payouts determines the payback percentage of the machine.

They could make each 7 symbol programmed to come up 1 in 10 spins, 1 in 100, 1 in 1000, or even 1 in 1 million if they like - doesn't matter.

__________________
Slotty McSlots