Michael's shameless and sudden move made both Leo and Iris pause for a beat, not keeping up with the situation.
Leo had been around Michael for a long time too, so he knew Michael was... special. It was true Michael was afraid of Iris, but his behavior would swing from scared to way too comfortable, like the two of them were actually good friends.
Maybe Michael's dizzyingly fast attitude changes were also a big reason Iris found messing with him more fun than Leo.
Thinking about past grudges, in her mind, Leo and Michael were about equally guilty. But hitting Michael felt a lot more interesting than hitting Leo, so at some point without realizing it, her plan had changed to just wanting to beat him up.
This difference could be seen as a kind of cultural and experiential gap. Michael back in the day hadn't carried any responsibility, had no ideals, and was a pretty smart guy but a total drifter who did whatever he felt like.
