Why does preconditioning finish way before set time?

I have a schedule set for preconditioning, but the car always seems to be done 15 minutes before the time I set. Why can’t the car just be done at the time I want? Not 15 whole ass minutes before. Maybe I’m complaining for nothing, but it’s a little annoying. Like why even set a time if it’s gonna b. done way before you’re gonna get in the car