One person's gut feeling is nothing more than anecdote as opposed to evidence, which is admittedly difficult to collect either way on something like this. When you're talking about high variance games like slot machines it is very difficult to draw any conclusions based on a single player's gameplay.
The perceived reduced frequency of big wins (or screenshots of them) on older games could easily be explained many ways: US players no longer playing, loads more new games to choose from, players bored of the old games hence playing them less, many of the older games aren't available on the newer browser-only casinos, your own confirmation bias, etc.
I don't buy your fictitious example of how they calculate RTP because if there were 10 slots that returned the RTP numbers you say, the average would not be calculated like that. Instead it would be weighted according to how much was wagered on each game. Individual games probably don't even come into the calculation - I would expect it would be a simple calculation of (Money won / Money wagered) over a month on each game class they report on. Go have a look at 32 Red's ecogra payout reports for every month last year and you will see that the slots return roughly 95-96.5% every single month. This is very consistent and as they are one of the largest Microgaming operators, and don't have their gameplay split up among countless smaller properties that all report their numbers separately, you would expect it to be indicative of all Microgaming casinos.
I know Redbet and BetAt do publish the theoretical RTP numbers for their Microgaming games on their website. I'm sure there are others that do this.
In answer to your last question, there is no way to prove whether the RTP numbers are set or not, or even if they have changed over time in the past when no-one reported the individual theoretical numbers. Not unless you worked for Microgaming or a casino I guess. But if someone at Microgaming told you they never changed, you probably wouldn't believe them. But if they told you they did change, you probably would.
The thing that always puzzled me is why online players expect the RTPs to be set in stone. They aren't in the land based casinos. The same game on separate machines sitting right next to each other may have different settings. It is well known that the payout rate is lowest on penny slots, as opposed to the high roller ones. You put in your money and you take your chances.
All I can say is if you feel this way, you should stop playing at Microgaming casinos as suspicion is no good for your health!
The perceived reduced frequency of big wins (or screenshots of them) on older games could easily be explained many ways: US players no longer playing, loads more new games to choose from, players bored of the old games hence playing them less, many of the older games aren't available on the newer browser-only casinos, your own confirmation bias, etc.
I don't buy your fictitious example of how they calculate RTP because if there were 10 slots that returned the RTP numbers you say, the average would not be calculated like that. Instead it would be weighted according to how much was wagered on each game. Individual games probably don't even come into the calculation - I would expect it would be a simple calculation of (Money won / Money wagered) over a month on each game class they report on. Go have a look at 32 Red's ecogra payout reports for every month last year and you will see that the slots return roughly 95-96.5% every single month. This is very consistent and as they are one of the largest Microgaming operators, and don't have their gameplay split up among countless smaller properties that all report their numbers separately, you would expect it to be indicative of all Microgaming casinos.
I know Redbet and BetAt do publish the theoretical RTP numbers for their Microgaming games on their website. I'm sure there are others that do this.
In answer to your last question, there is no way to prove whether the RTP numbers are set or not, or even if they have changed over time in the past when no-one reported the individual theoretical numbers. Not unless you worked for Microgaming or a casino I guess. But if someone at Microgaming told you they never changed, you probably wouldn't believe them. But if they told you they did change, you probably would.
The thing that always puzzled me is why online players expect the RTPs to be set in stone. They aren't in the land based casinos. The same game on separate machines sitting right next to each other may have different settings. It is well known that the payout rate is lowest on penny slots, as opposed to the high roller ones. You put in your money and you take your chances.
All I can say is if you feel this way, you should stop playing at Microgaming casinos as suspicion is no good for your health!