Because pixel is square thus you are only looking at a representation of a circle not the actual ring in material.

No, its the LCD technology, but what is a true circle? A needle point mechanical pen drawn with a circular stencil on smooth paper are not perfect either, try use a magnifying glass to view the line and double that compare to the width you see on LCD monitor, now how can it be smooth?

Or, you think glass surface is smooth but try look at it with electro microscope, nothing is perfect even light bends by gavitaty. So what is the point?