I thought it was 1.5 levels.

"Bubbles" have no meaning TMK other than something players came up with based on the skinning of the increments in the default UI, which are only there to make it easier to see your progress. It's for psychological effect so you feel like you're accomplishing something, while an undelimited bar would be less distinct.

Did you do anything while you weren't grouped or while the level delta was too great? That would eat rest instead.