RTP is the game’s theoretical average return over a very large number of plays;...

https://astro-wiki.win/index.php/How_Do_I_Read_a_Slot%E2%80%99s_RTP_and_Not_Overthink_It%3F

RTP is the game’s theoretical average return over a very large number of plays; the house edge is the share it expects to keep. A 96% RTP means a 4% house edge, not a promise that you’ll get 96p back from every £1 you stake