I've flown with airlines before where there was a cascading delay due to a "plane deficit" at the terminal (not the technical term, that's my own). Not to say it's always uptime, but I imagine there are instances of constant uptime.
They can't just change things up on a dime like that. Even if it's 3 AM and most planes are sitting on the ground they can't just be used for your flight like that because they are all scheduled to take off in the morning rush a few hours later.
