MacBook Pro: water damage improved screen's brightness!
The reason it's brighter is because water has a refraction index that is closer to 'glass' (as in the specific material used in the screen--I know it's not really glass per se) than air does. This means that less light is lost in the area between the LCD and the front glass (or between the LCD and the backlight--but not very likely) and thus the screen appears brighter.

You'd probably get a better explanation on Physics in terms of specifics, but this is the answer in a nutshell.