As far as I know, matted means, a silly piece of 
carton board has been added to make the cel look 
prettier framed and to suck more $$ out of the 
buyer. It's a way for sellers to add value to 
what they're selling, boost shipping fees and 
stick their tongue out to the buyer who paid 
maybe more than the cel was worth cuz it 
was "matted". You get those in art shops and so 
on beginning at $4. Means, no need to pay extra 
for things you can get behind every and each 
corner...

Ore-sama