Of course the hash when you click the button is going to pertain to all previous rolls but if you dont click the button you dont see the hash, when you click it then the site can chose a hash that pertains to all previous rolls and you would think they are legit
I doubt any dice site has that kind of computational power no matter how much they've earned. It would be like trying to use vanitygen to get 1UNPOSS1BLEADDRESS1234567890 and expecting it to happen instantly.