Everything on this site is arithmetic you can reproduce. This page says exactly what the simulator does and, more importantly, what it does not claim.
Each spin returns nothing, a small amount, a mid amount, or a rare large amount. The three return classes are weighted so that their expected value equals the return-to-player figure you type in. Hit frequency sets how often any return occurs; the split within it is fixed at 80% small, 18.5% mid, 1.5% large.
A distribution with the right mean and roughly the right shape — skewed, long-tailed, most sessions below average. That is enough to demonstrate the gap between the advertised average and the typical session, which is the only thing we are demonstrating.
It is not a model of any particular machine. Real paytables have dozens of outcome classes, feature rounds with their own distributions, and return contributions that differ wildly between titles. Anyone claiming a simulator reproduces a named machine without its published paytable is guessing.
Because we would have to take them from somewhere. Manufacturer figures are not uniformly public, jurisdictional minimums differ by state and change, and second-hand tables circulate with no provenance. We would rather publish a method you can check than a table you cannot.
The simulation runs entirely in your browser, in plain JavaScript, on the front page. Open the page source and the whole model is about thirty lines.