Sharon, 

As per the lesson you linked, the adjective modèle is the same form in masculine and feminine because it ends in a mute ‘ e’. 

With any adjective that ends in a terminal mute ‘ e ‘ in its ‘default’ form, such as ‘ modèle ‘, an additional ‘ e ‘ is not added for the feminine form, and the masculine and feminine forms are identical to each other, in both singular and plural forms ( with the addition of an s in the latter of course ). 

By extension from the ‘rule’ applying to adjectives ending in a mute ‘ e ‘, adjectives ending with ‘ é ‘ in default form, will take an added terminal ‘ e ‘ in feminine form. 

Just the way it is. 

Adjectives ending with mute -e don't change in the feminine in French

https://www.wordreference.com/fren/mod%C3%A8le