Runoff estimation by Soil Conservation Service Curve Number

The Soil ​Conservation ​Service Curve ​Number (SCS-CN) ​approach is ​widely used as ​a simple method ​for predicting ​direct runoff ​volume for a ​given rainfall ​event. In SCS-​CN, the ​derivation of ​the equation ​starts with an ​assumption: ​

Runoff/​Maximum ​possible runoff ​= Storage in ​catchment/​Maximum ​possible ​storage in ​catchment. ​

I have not ​come across any ​book explaining ​the ​logic behind ​this primary ​assumption. I appriciate if any one can explain me this logic.