I believe that this is true because generally burgers are made of meat which is often more expensive than potatoes. Assuming Burger Stop serves burgers and Fry World is more of a chip truck you would assume that people would spend more money on a full meal than just fries. The price equivalent would also be relative based on the cost of making fries versus burgers. Also, you would assume someone heading to a place called burger stop would be more likely to be looking for a burger AND fries rather than someone heading to something called fry world.