It is because your brain seeks contrast over accuracy. Thus, it each optic neuron samples the experience of the neurons right next to it, and alters its’ output so that the contrast is highlighted. Another experiment you can do is draw a series of black boxes set in continuous lines of white (so like a city map where the blocks are black and the streets that surround them are white) If you stare at the intersections long enough, you will begin to see darkening in the intersection, because the light-dark sensing optic neurons (rods or cones? neuroscience was a long time ago) are sampling from their neighbor neurons, which are picking up the image of the dark spots nearby.

Ugh, that was hard to explain (if I did at all). Someone with more time and google savvy can find a link to someone who’s taught this, instead of simply learned it ten years ago.