Yeah, I heard about that book - ok I got it. I actually learned something today.
You can choose to have a blue label and effect the grade or have the green label and the grade be based on the books condition, on a low grade book, a missing coupon rolls into the number and/or type of flaws allowable for a particular grade. i.e., a missing coupon is, after all just a large piece of missing paper - which is within the criteria for a 2.0 I would guess.
The idea being on a higher-grade book it's better to take the green and have the books grade labeled incomplete and based on the remainder of the flaws whereas a low grade book can "absorb" the flaw and still be the grade it looks.
Aye carumba, my heads hurts