I remember thinking the iPhone 5 screen looked darker than the 4/4S because they changed the glass touch lamination a bit from how it was in 4, which made it the most color accurate screen of the time, but looked darker. Eventually it became clear it was the richer colors and less backlight wash thanks to glass having little effect on the visuals anymore.
So my guess is that the lack of light and glass obstructing the rich colors makes it seem darker, but really it is just more accurate.