I read that the word "Professor" means "Highest grade of faculty" in some contexts, and is a general term for many types (Associate Professor, Adjunct Professor, etc) in other contexts. Why would we take the term for the highest form of something to be the general term? Is there a name for doing this?
It is a form of mild, innocuous flattery.
A Mayor-Elect (elected but not inaugurated yet) is still addressed as "Mr. Mayor."
Lieutenant generals, major generals, sergeant major generals are all addressed simply as "General" for the same reason.
And even veterinarians, dentists, psychotherapists and some botanists are addressed as "Doctor" by actual doctors. The opposite would be to address a real doctor as "Apothecary."