Why do pictures in the background or window often appear distorted if you scroll them up or down 1 pixel each V-Blank ?

This is because all versions of GB (i.e. GB,GBC,GBP,SGB,...) have interlaced screens and are redrawn 30 times a second. (i.e. All odd lines are drawn in one screen refresh, all even lines are drawn in the next screen refresh.) As a result, if you scroll a picture up or down 1 pixel each V-Blank you are seeing every other line twice during scrolling. (Info of GBC interlace from Pan of Anthrox.)

The GB screen isn't a true interlaced screen. It appears that every line is redrawn with every screen redraw but every other line is just much dimmer.