I stayed at the University of Montreal student housing, situated on the side of a hill. My room was on floor 17, but the building numbered its floors starting from 7 for the ground floor! Why 7?
Here is my explanation: There were 140 steps from my room to the ground floor: 14 steps per floor. Then, to go to the bottom of the hill I had to go down outdoor staircases, and counted 106 steps, meaning about 106/14 = 7 floors. So, my guess is that the university numbers floors starting from 0 for the bottom of the hill (where another student housing building is located). For my building, it constructs an imaginary extension down to the bottom of the hill and starts counting from there. In other words, the floor numbers are a substitute for altitude: they are not relative to the ground level of the place where the building stands, but have an absolute meaning.