It is not odd anymore, some studios are starting 
to make laser copies of their original 
backgrounds, then sell them with the cels 
together.  Actually, the studios that using the 
traditional cel animation still hand-painting the 
backgrounds, not painting on computers as some 
people thought.  

As you know, one sequence of cels usually have one 
background.  But if they make laser copies of the 
backgrounds, then sell them together with the 
cels.  The cel prices can go higher because they 
look better with the backgrounds, even though 
they're laser copies.  You see, the start or end 
sequence cel will still get the original 
hand-painted background, but the other frames like 
A3 will get laser copies.