Because normally, in Japan, japanese uses rice 
paper to back cels. Paper made from rice. They 
are thinner and more porous than glassine as what 
Tralfaz mentioned.

They are different in texture, thickness and 
material. Rice paper is more easy to tear, looks 
glossy on one side and matt on the other. 
Glassine is glossy on both sides I believe.

Bend a piece of rice paper and a piece of 
glassine, I'm sure there's a difference. You can 
also wet it by dripping a few drops of water on 
it and then dried it out. Rice paper absorbs 
water faster than glassine.