What's going on here... (use browser zoom to really see)

The shape is a stack of boxes of different widths. If the width of the boxes is less than a pixel, they might fall "in between" the pixels. The wider the box, the less likely this is to happen.

As the stack moves from left to right, the number of boxes that cover the centerline of the pixel gets bigger, so the stack looks taller.

On the right, there is a "zoom in" - the white line shows the "center of the pixel". The blue stack is 5x wider, but exhibits similar artifcts.