I'm not Yann, but from what I know about 
lithographs, the un-numbered set are usually 
artist proof and screening proof. There price are 
usually less than the signed and number group, 
since how many of those released are unknown. They 
are also semi-questionable, since they usually are 
supposed to be destroyed or may have printed 
without the artist or in this case the animation 
house's permission.

Unnumbered lithos can become more expensive 
(especially with artist proofs) than the number 
edition, if they show some interesting changes 
from test prints to the final printing. My dad has 
several artist proofs of etching done my Rembrant 
and in one case shows a complete change of the 
layout from the initial test print to the final 
produced etchings (he scratched out one of the 
figure are replaced him with a wall to better 
balance the image).

This may not be relevent to these cels, but that 
about the only basis that I know of for examples 
of unnumbered artwork versus numbered artwork.

Marcus