Have you ever tried holding a cel up to the 
light, a normal cel? You see how the light shines 
through? Since that's all they needed to prevent, 
they don't always go to the edges, but it's for 
the same purpose. They don't care about being 
meticulous because on camera you don't see the 
paint. And sometimes, yes, they do leave the 
edges unpainted for a "glowing" effect. It would 
be too much effort for a company to spend money 
on painting the backs of their cels whenthey 
weren't even going to use them anymore ^^;; Just 
my 2 cents ^^v

Ja!

-Misha-chan