As explained above, RTP is actually given by the games provider, not by the casino. For example, if you're playing Evolution games, then it's Evolution giving the RTP, not their casino.

As Karl explained, this is all based on RNG, which is actually coded into the game. Then you have 3rd party people to come and check on all these providers, testing their software and making sure the RNG is just like they say.

That's the beauty of regulated casinos, that need to use regulated providers