I'm replying to this again because I just found the explanation that was posted on their Twitter page yesterday.  The RTP is calculated over long sessions of play and fluctuates in shorter sessions. This basically means that the website calculates what was the return to the player on average in the last one million spins and then shows live results on their website.

It's also clearly stated on that when RTP is for example 99%, players shouldn't expect to get 99$ back after they wager 100$, 99% RTP simply gives player back the 99% of money during the long-term. e.g. if someone plays actively and makes a million spins in slots with 99% RTP, they'll receive back roughly 99% of what they spent.

Guys, yesterday someone also won $15,000 MAYC on Metawin. Their rewards and giveaways are super for a newcomer casino on Bitcointalk.