why does ace call the doctor professor?

why does ace call the doctor professor?

Initially a brash and reckless teenager, Ace matured significantly into a strong, independent adult over her many years of TARDIS travel and other experiences, who was capable of surviving on her own. Born on Earth, Ace was a central player in fighting the return of Fenric and battled many of the Doctor’s most infamous foes alongside him, including the Daleks, the Cybermen, and the Master. Her timeline was complicated; during an encounter with the Quantum Anvil, she saw her future fracture into various different but all valid possible outcomes. Amongst these futures were continued travels with the Doctor, a life in 1887 France, and becoming a Time Lord after attending the Academy on Gallifrey.

why does ace call the doctor professor?

I have noticed in some of the later Sylvester McCoy episodes how Ace calls The Doctor “Professor”. Why does she call him this and not “Doctor”? No other companions ever called him anything else but “Doctor” and I was wondering where “Professor” came from.