I would not trust this site/bot. They claim that your roll comes from the most recent Bitcoin transaction, but the Bitcoin network receives many transactions per second, and the time for them to propagate to all nodes can be multiple seconds. Therefore every node receives transactions in a different order, and this site could simply pick a number that benefits them. As there is no way for you as a user to definitively see the number they came up with was non-biased, THIS GAME IS NOT PROVABLY FAIR.