It is a matter of convention.

Using design tools like the Datcom which is a set of tools and charts that let you use a sliderule (bamboo calculator, no batteries) to design an airplane, the authors had to make a decision. By including the fuselage they could develope a set of tools for the wing. Then modifications to the pure wing characteristics due to the fuselage were another set of charts.

It is like any science, engineering, rock throwing or pie baking thing. It helps if everyone is talking the same language.