Its a known fact that some casinos take liberties when it comes to currency and there isn’t much we can do about it. They will use the excuse that it is too much of an overhead to use spot rates when making payments which is total nonsense. The payment methods are even worse, with Neteller particularly bad, especially converting from Euros for some reason.
It’s a great opportunity to try and make some cream and it’s too tempting.
That’s a big discrepancy you have though. I would ask for an explanation.