Photocopiers do this too!

Moving the paper part-way through making a photocopy produces the same effect, because the moving light in the photocopier builds up a copy one line at a time.

If you move the paper slowly in the same direction as the moving scan, the copied image gets compressed.

(In fact if you move faster than the scan, the copy actually gets printed backwards. Try it!)

If you move it in the opposite direction, the copy gets stretched.