Payouts are a matter of probability.
There are 256 reel stops (most are blank)
Three random numbers are drawn (RNG) and each reel stops at what ever position the number dictates. The spinning is just for show.
There is an internal PayTable .. 3 bars gets you xxx, 2 cherrys gets you yyy
The %% payout is based on the probability that the numbers match up with the internal paytable.
For a looser slot - The number of internal entries that have payout increases and a tigher slot has fewer
... however the 3 numbers are random and the payout %% is not forced.
For instance if you have a coin - heads or tails. 50/50 chance. You could get 100 heads in a row. The probablility of this is low but maybe you get 80/20.
If you keep flipping - 10million times. you will find that it will balance out to pretty much 50/50 and even if your off by 4000 in either direction - mathematically against 10million spins its insignificant.
The Nevada gaming board certification is that the slot is run 7 times, for 10million spins, the %% stated by the slot manufacturer much match the actual outcome 5 of 7 times with no more than a 1/2% point in either direction. Lots of math involved. This is why you see games like Double Diamond continue to exist from the M, S, S+, S2000, AVP, etc and then there different themed games that are clones.