I just started noticing this...whenever more than a certain amount of screen is taken up by something dark, like these forums or steam or a dark movie scene, the backlight OR the brightness settings (It's hard to tell) are instantly (a smooth 1 second transition) adjusted to make it even darker.
Here's an experiment I did: on a dark grey background, I opened an empty folder (which is white) and the brightness/backlight of the screen was at maximum. I started shrinking the folder window and once it got about as small as a 2x2 windows 8 icon, the screen VISIBLY darkened, making the white of the folder and the fonts on the taskbar somewhat brown-greyish.
This is a bit annoying because it's noticeable AND because it makes small white details darker, while I prefer high contrast. Does anyone have any clue what might be causing this? Adaptive screen brightness is disabled and there's nothing indicating this function on the nvidia control panel.
ASUS G750JX -- i7-4700 - 16GB - 128GB 840 PRO SSD + 1TB HDD - Nvidia GTX 770M - 120Hz 3D glossy display