I think Harry very likely has hit the nail on the head with his comment on brightness. Most backlit screens are generally multi-purpose and used for instance to read comics and watch movies. Higher brightness levels make the colours "pop" and gives more life to movies and such and when the time comes to read a book how many think of adjusting the brightness?
Having said that, I think Ankh is right on the money too when (s)he (?) says that "Everything about human-machine interaction is complex". Human responses to stimuli are often inexplicable, or can only be explained partially, in scientific terms. Just visit a Hi-Fi forum sometime, to witness protracted discussions/violent arguments over which piece of cable is better suited to connect two audio components.