What people aren't mentioning is how small the ring was. A small ring is a bit like a shit pitch in football. It can be a leveller.

You have less room to set yourself, and your opponent is constantly on you. Technique can go out the window as close combat takes over. I honestly think that's what happened with Kessler. Plus i agree with what Miles said about Kessler stepping up in class. It's similar to how Hatton started looking once he started to fight decent guys who were in their prime.