Less is sometimes more costly

I realize I am not the wisest business person around. But perhaps somebody who knows business well can explain the following to me?

This morning, I stopped at Burger King (or as Clem Haskins once called it, Burgers King) for a quick breakfast. I chose No. 2, a Biscuit with sausage and egg, tater tots and coffee. List price: $3.69.

The guy at the counter then asks the standard question: "Would you a large version of this?" I said no.

He then asked if I wanted a small or large coffee. I told him a small would do nicely.

He rings up the bill and says this will now cost $5.31. Seems that it costs a buck or so more if you order a small coffee instead of the medium you usually get.

I told him to forget the small coffee and go back to the original order. That meant he had to get a manager to void one order and start all over.

I leave it to smarter people than I to explain why this is so.