You do understand that people advertise apartments available for rent before the previous tenant has moved out...
In Québec, the majority of leases end on July 1st and you have a number of people moving on that day which is quite greater than the vacancy rate. You can sign a lease for a new apartment months ahead of time.
It’s similar to the unemployment rate, we call ~4% unemployment “full employment” since it would be impossible for every single person to have a job in a fluid job market.
True, but sometimes people just leave without giving warning, or the next person can't move in immediately. Sometimes, in a limited supply market, the landlord will wait for someone willing to pay a specific price since they know there's no other options.
In San Francisco, that's the case for roughly 5% of the units at any given time.