The game client/server is hosted by the provider, not the casino.How does the game client know that it's talking to a bona fide server ? Online MMORPGs are rife with 'private servers' that offer great customisations (and no subscription fees), why not private servers that can demonstrate inflated RTP by returning a higher percentage of winning spins back to the client ?
(or maybe I just want the buzz of awarding myself a wildline on the first bonus spin)
Casinos(unless they run shady pirated software) do not have access to changing anything, neither do they have access to any maths or source code.
If you take netent games for example, you will always see them connect to ****.casinomodule.com