Someone explained this in a prior thread, but it's like this:
- hero powerup: breaking your limiter
- monster powerup: wishing to be something else (stronger, faster, etc)
Saitama broke his limiter repeatedly, without wishing to become something else. He's like a Saiyan that did zenkai a lot of times.
Garou broke his limiter a few times, but in his fight against Saitama, he compromised and ended up becoming a monster. Saitama said so himself. He said "it was a mistake to lower the hurdle right before the goal". So to defeat Saitama, Garou wished to be something else, and thus became a monster.
I guess him coming back from the transformation means he accepted himself as he was (really).