A previous study presented a very simple model mainly based on Sanderson's principle for estimating important reactivity descriptors, such as the local hardness and the local electrophilicity. In another study, a new way of calculating the Fukui function was obtained that resulted in a new operative formula for the function. We also obtained the second-order partial derivative of the electron density with regard to the number of electrons. The current study analyzes the relationships between the two models and justifies the origin of these relationships.