The actual stops on a slot machine reel correspond to a computerized or virtual reel with stops, which is what determines the chances of hitting a particular symbol combination, and consequently, the big jackpot. Since the virtual stops correspond to a fewer number of actual stops, the odds of each symbol combination are determined by the number of virtual stops per actual stops.
A regular slot machine will have only one virtual stop corresponding to the top jackpot spot on the actual reels. For a machine that uses the value of 32 to process RNG generated sequences the chance that the jackpot image will land on one reel is 1 in 32. For all three jackpot images to line up on all three reels (with all three reels set up in the same manner), the odds of that happening would be 32x32x32 = 1 in 32,768.
For those online casino slots that pay larger maximum jackpots, they will have an even greater number of virtual stops (64, 256, 512), thereby decreasing the odds of winning. What this means is that the odds of hitting a symbol combination are not based on the amount of actual stops on the reels, which is what many slot players fallaciously believe.
And while slot machine games used to be programmed so that the blank stops above and below the jackpot symbol would correspond to more virtual stops than other symbols (so that near jackpot misses would often show up and trick the player into believing they are getting closed to winning the jackpot), this is no longer allowed in well regulated gaming jurisdictions like Las Vegas.
As for a slot machine's RNG, which ultimately determines the payout percentage, it is tested by gaming regulators to insure it is paying back a minimum percentage of funds that are deposited into it. A machine with a payout percentage of 95 would be paying out 95 cents for every dollar put into it by bettors. Even with only a 5% profit, it is easy to see why the casino never loses.
In highly competitive gambling markets like Las Vegas, Macau and Atlantic City, the payback percentages are often going to range from 90% to 97%. Since there is greater competition, not to mention strict regulation laws that require a minimum payback for each machine, players have good chances of finding looser slot machines than normal.
Since each machine is regulated by a computer chip (which the odds are programmed into) the only way (in most cases) for a casino to change the odds of a machine would be to change out the chip, which contrary to some slot myths, does not happen in strictly regulated areas. In other words, the "tightening" of slot machines simply does not take place.
Likewise, slot machines do not get "loosened" the more one plays them either. Because of the irregular sequences of numbers created by the RNG - which are as equally random every time the reels are spinned - a player always has the same chances of winning every time they play. It is not that a machine is "ready to pay" at any particular time, more so than it is the luck and timing of spinning the reels at the exact millisecond the right random number sequence is generated.
