An explanation just occured to me....
....could it be that your monitor runs 640x480 as an interlaced display mode? Most of the time the scanline effect is the result of an interlaced image, IIRC.
I might be getting my terminology mixed up here (and as a result, may completely butcher what I'm trying to explain), but this is my understanding as far why an interlaced display mode could produce scanlines:
Most monitors run using a non-interlaced display mode, which means they draw the entire image to the entire screen during each display update (i.e., one "refresh"). This is different than what analog TV's do, in which case they draw only half the image to the entire screen during each refresh. Note, though, that when I mean "half the image", I mean "every other line of pixels". Thus, at any given time we see an image with black lines after each line of pixels being displayed. And for some reason, they call images displayed in such a way as being "interlaced".
Now, I've heard that there are some monitors out there that support certain resolutions as an interlaced mode, and others as a non-interlaced mode.....though I heard this a long time ago...