Theoretical RTP is the calculated amount returned to the player(s) versus the amount wagered by the player(s) over a large number of spins, represented as a percentage. By large we’re essentially talking infinite, however the more spins that are made, the closer the actual RTP will be to the theoretical RTP described.
What slot were you playing on, out of interest? Because that is 4% RTP over 100 spins, which is pretty abysmal.