It is using a montecarlo simulation to generate probabilities, not probability math. It's using a somewhat small iteration count for snapiness (and tablet support), but you can modify the URL where it says montecarlo=100000 and up this to a larger number of iterations. It will take longer (potentially much longer depending on how big you make the number) but will get your results closer to the actual mathematical probabilities. Note that if you change the number you should hit enter in your URL bar to actually have the change take effect.

Consider it a test of your browser's scripting performance