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.