[xp84]

Same thing happens watching something projected on a screen (in a room of non-zero brightness). Imagine a zebra or a checkerboard projected on a white screen. When you're looking at the projected image, you really see "black and white" but only when you consciously reconsider and look at the margins of the screen outside the image, you realize the zebra or checkerboard's "black" parts are actually full white (possibly even a touch brighter due to LCD) and the white parts are just ultra bright white. It always amazes me that we're naturally able to perceive any (reasonable) relative gradient of dark to bright as though it's true black-to-white.