Often, one background is used for muliple cels
and/or scenes.  So someone may make photocopies
of that background to go with other cels in 
the sequence... 

I've found that some galleries (in the US that
I know of) seem to like to make copies of the
original handpainted background and put those
copies with the cels, then sell the background
as a separate piece of art... so they can make
more money from it.  (Which is a shame to see
pieces of a production broken up, but what can
you do?)  I have some FernGully cels that have
copies of the original backgrounds (with the
production numbers and everything) that are really
nice glossy laser copies... but they're still
only copies.  And I KNOW (because the owner of the
gallery where I bought them told me) that the
cels came with copies because the gallery sold
the original backgrounds a long, long time ago.
;_;

Also, with new DVD capabilities, people are 
learning how to print out screencaps and edit
to just have a background to go behind their 
cels... basically just to make them look nicer
or appear more presentable.

Peace!
Kim
aka Lady Zatara