Return to Player (RTP) is a theoretical long-run average calculated over millions of simulated spins, not a guarantee for any individual session.
A 96% RTP slot returns, on average, $96 for every $100 wagered — over a large enough sample size. In a single 100-spin session, results can (and regularly do) swing far outside that average in either direction; that spread is what volatility describes.
When comparing games, RTP is most useful as a relative signal: a 97% RTP title is mathematically better for the player than a 94% title from the same provider, all else equal. It says nothing about how bumpy the ride will be to get there — for that, check the volatility rating alongside it.