The spins necessary for you to get your stated RTP are not fixed but a simple representation of the average - they cannot be fixed or the game would not be random, would it? You could have 100 spins at a Euro and win 96 back, hitting it in just 100 spins. Equally you could have 1m spins and be running at +/- say 20% deviation from the stated RTP, the deviation likely to be bigger the higher the volatility. I sense you are asking that if the RTP was say achieved over 1m spins, you had had 990,000 and were running 10% below the stated 96%, would you be guaranteed a 'recovery' spell over the next 1,000 spins? No. You wouldn't.
All the developers can do is audit the game over billions of RNG pulls and check it does fall very close to the RTP the maths was designed around.
Most games actually DO give the customer and sometimes the player the average win frequency, feature frequency etc. in the crib sheets/rules, even average feature returns.
True, you can get predetermined feature results (Netent you say, also like Push Gaming's Jammin' Jars) whereby the RNG result is from say 0x to 1500x and all you watch is animations to demonstrate the predetermined outcome. Some, like
BTG, profess to pull random outcomes for each free spin therefore the total is not pre-decided as soon as you land the feature. Both methods are permissible and legal.
The RTP is sourced from the RNG on the game server, all players everywhere pull results from it. There can be more than one server for the game, depending on whose platform it is provided through, but it's irrelevant to you personally. The results aren't 'pooled' as you suggest when you say that a player winning 40,000x would mean other players have less chance. The 40,000x winner has simply picked the luckiest 1/10million RNG result, and that 'ball' goes straight back in the RNG pool so you have exactly the same chance of picking it on your next spin, as do all the players including the person who just won it! In fact if you refer back to
@trancemonkey he will tell you that IGT actually go to great pains to specify that particular point in their game rules.
There's no point in 'compensating' or rigging the pool of RNG outcomes as the server is providing huge quantities of results to thousands of players and the game will, eventually, return it's 96% RTP and 4% house edge to the operators. Of course, some operators may 'lose' on the slot if their players win and be running 105% on it whereas another may be running 91% and have a temporary 9% house edge - the gambling aspect affects both you and the casino! You can see this demonstrated at casinos like Videoslots who state the current RTP for their individual slots offered.
So no, the server doesn't cater to the individual player, nor the individual casino (unless the casino is the only operator using that particular game server).
