How the free roll numbers are calculated are available on the site. From the Free BTC menu click on "Provably Fair" and you'll get the detailed information
Its a total of 10,001 (0 - 10000) combinations so the chance of winning the jackpot is 0.009999%.
I think wetsuit's mistaken there.
All possible calculation results before rounding off lie between 0 and 10,000. Out of these possible results, everything larger than (and including) 9,999.5 is rounded up to the number 10,000. The chance for getting 10,000 is therefore 0.5 in 10,000 or 1 in 20,000.