Perhaps McNuggets are normally sold as combo meals, and the whole purpose of offering cheap 20-piece nuggets is upselling people to a combo with better drink ma

Perhaps McNuggets are normally sold as combo meals, and the whole purpose of offering cheap 20-piece nuggets is upselling people to a combo with better drink margins. In that case, the relative profit between a 10-piece combo meal and a 20-piece combo meal could be about double in the latter. Perhaps raising the 20-piece prices deceases the likelihood of an upsell.