Just cause a site offers provably fair verification doesn't mean it actually is. It's been proven through someone creating a site showing it that if the server can predict a trend you use such as always betting low or always betting high, a rigged server can give rigged seeds assuming that person doesn't change their pattern or their client seed.
I think I know what site you're talking about. Someone (vlees, maybe?) made a site which brute-forced a server seed from your client seed to get you exactly the roll that would make you lose. His point was that you need to pick an unpredictable client seed, or the server seed can be chosen to hurt you.
Almost every site these days uses the same server seed until the user asks for it to be changed, which renders this attack moot. Just make sure you pick an unpredictable client seed *after* noting the server seed, and the site can't mess with you in the way you're describing.
Just to confirm, we are using the exact method Doog has pointed out.
Hey dooglus why are you always defending this site?
Are you in anyway connected with it?
You are here everytime. Even more than Safe Dice (user) himself.
Its because you guys asked and mentioned him so many times, and he does online very often (that's why he got his legendary rank in this forum). And as my friend, frank has pointed out above, we are not in anyway connected to Doog. Things would have been much easier if we do, but we don't.