Number of rooms. The more rooms cleared the higher the bonus will be, even increasing in percentage depending on the number.
Does the bonus only start once you've had a combat? I experimentally went into a dungeon, had several "search the rocks and bushes" nc's, and then left when I ran into my first actual enemy; I had 0 bonus xp despite the fact that I'd technically cleared several rooms.
also is that why the bonus xp is sometimes different for the same amount of rooms?