This is exactly what McDonalds stores in the Seattle market do. The 10 piece version is $4.59 ("regular price") while the 20 piece is $5. They even package th

This is exactly what McDonalds stores in the Seattle market do. The 10 piece version is $4.59 ("regular price") while the 20 piece is $5. They even package the 20 piece as two boxes of 10 pieces each. I commented on it to a store manager--he and I chat a lot, I eat at McDonalds far, far more than I should--and he said that franchisees do it because corporate rebates them a ton of money for the promo price, company-owned stores do it because that's what corporate tells them to do, and corporate does it for a reason that is unknown to anybody outside of Oak Brook, IL.