The difference is due to the way that FBC rounds the number. I believe it was TheQuin who actually explained how it works once, but basically he said the chance of spinning a 10000 is only half of how it appears... I can't find the exact post that explains it but this is why the $200 win happens disproportionately less often than the $20 win.
That means
anything under 9999.5 is rounded down. I accounted for that in my post above. There's still a factor 12.5 unaccounted for.
"The way that works is that the maximum integer from the first 8 characters of the hash is 4294967295 and they divide that by 429496.7295, which results in the largest possible number being 10,000. The key here is that they ROUND OFF the result, so if you roll 4,294,967,294 (-1 from 4,294,967,295) you get ~9,999.999998, which is then ROUNDED OFF to 9,999. So that makes the only way to hit 10,000 to roll 4,294,967,295."
https://bitcointalk.org/index.php?topic=655762.msg54473263#msg54473263by hejag93133 May 21, 2020, 12:14:42 PM