Auto Brightness in Mountain Lion
I see that you don't have the "Slightly dim the display while on battery power" setting enabled in Energy Saver, but do you happen to have "Automatically adjust brightness" enabled in your Display preference pane?



That's the setting that adjusts your brightness based off of the light level detected by a Mac's ambient light sensor. (You may have already thought of this and or maybe it's too obvious but I wanted to suggest it just in case.) Good luck!