Here's the point,
I've read somewhere here (
https://bitcointalk.org/index.php?topic=5383777 ) that RTP may vary from casino to another and can be adapted to the casinos wants ! In addition in some casinos software is executed under (the website/casino URL), normally it should redirects to the game provider servers, so you know you are running the game from the original makers, but running the game under another (Interface if we could say) there's no more, or any insurance there, and everything can happen.
I agree with you in that, and that's why we should be careful when we select a casino to gamble our coins, If you know that some casinos have a variable RTP just stay away from them and share the message with the community. We always appreciate any warning about new scams schemes.
As I mentioned in the first, for me if a site has an RTP variable, that only means it's rigged. I'm not saying users will never win, I say the house will always win and no luck will be involved in that.