Lets try a simple polygon. The above polygon is a rectangle which consists of two horizontal edges and two vertical edges. Shown below is the same polygon with scanline 7 filled in (scan converted). As can be seen the right edge of the polygon is not drawn, as explained in the Explanations section. For the same reason, scanline 8 does not have any pixels filled.
The complete scan-converted polygon looks like this:
The filled in pixels look slightly skewed with respect to the actual polygon that needs to be drawn. This is just an artifact of the exagerrated view of the pixels and scanlines that is presented in the above figure.
From Graphica -- see details 26.11.1996