I'm not sure but I remember reading a similar discussion back then as the player tried to point out there was one casino that had a slightly higher payout compared to another casino even though the game is the same.
I think one of the slot providers that lets you do this is Booming or Betsoft, not sure if it's both of them but i'm sure that the name of the provider is starting with the letter "B" since these are the providers that most casinos have back then.
I remember someone created a topic just like this before. Further discussions also is about if casinos can change the slots' RTP. Which most of the providers offers.
To add, if I'm not mistaken, online casinos' RTP on both DEMO and ACTUAL slot can be different, in which they add more RTP on the DEMO mode. Not sure if it's absolutely true, my experience on both gambling with real money and playing demo modes, in demo, I'm winning a lot. So I guess it is somewhat true.