I think OP thinks "won" means they cashed out and left/profit.
If I make 100 1BTC bets at 50% and win 50 of them , I "won" 50BTC but I have the same amount I started with.
750BTC won means they probably accepted ~1500BTC in total wagers.
They are that big no? Maybe I'm wrong. But I am 90 percent sure primedice is not faking their stats.
The thing is, the number hasn't changed for a long time. It's possible that they are faking their stats, but this seems very unlikely as they are very popular and trustworthy, and everyone knows that people bet a lot of money on PrimeDice, so there is no reason to fake stats. Probably a technical error, but even if not, I don't think we should worry about it too much as PrimeDice is still one of the most trusted bitcoin gambling sites.
I dont think primedice would do that and i know primedice is probably fair site thats why they are the most famous gambling site existing online maybe OP calculating the win/loss stats happen when betting and it turns that he lose when he bet big money in PD.
theres no need for them to cheat theyre earning big moneys in their site and also theyre giving big moneys as giveaways to this community.