You are correct - software can be changed & no doubt there are examples of where this has happened, which is why sites like this are here. (
Casinomeister's Philosophy and Mission Statement).
As the introduction on the previous link states,
"The bottom line for most people in this industry is to make a profit" so when you are creating games that already have a built-in house edge, there really isn't any need to 'tweak' it post release to improve that edge. (Surely if they wanted any games to be more profitable, they would have designed that game to have a lower RTP in the first place)?
In order for a game to be certified by Regulators, the software developer must confirm details of the game (inc those readily available, and those that aren't. (E.g. paytables, hit-rate, rtp, etc)). The Regulators approved testing house will then cycle the games to ensure that the stated information from the supplier matches the results from their testing. Assuming everything matches, then the game is approved for release, but that's not the end of it... Once a game is live, the Regulators are able to audit the games at their discretion. Most (if not all) of the reputable suppliers also belong to at least one of the industries many bodies that ensure fair-play (
). Operators that make the games available to the players also join these organisations and have monthly audits on the games performance to ensure that the statistics that the game produced in the aforementioned test cycle mirrors what is happening now the games are live, (
Old / Expired Link).
After the developers and Operators have gone through the above, (and knowing that their license(s) could be immediately revoked if anything untoward is discovered), do you really believe that they would risk their entire business (which could include sportsbook, poker, and bingo (as well as any other casino(s)) just to try to draw an extra % out of the RTP of a slot game?