When you're going to drive a trip in a car you don't normally use and either have your Driver ID with you or check in before starting the trip. What overrides what during registration.
In priority order: 1. Driver check-in (in the Driver app) 2. Driver ID (you have the tag with you) 3. Default driver (the one set as the default driver)
So if you lend the car to someone who has a fixed driver, but you check in as the driver before the trip starts, you will be registered as the driver on that trip/those trips.