I think there's some logic that says if you plug in after the scheduled time but still close enough, it still charges.
Imagine you have it set regularly for 12:05 am, and you usually plug in before midnight, but one day you go out and don't get back until 3am. Most people would still want the car to charge while they sleep in that scenario and would be pretty pissed if they woke up a few hours later with no extra charge. At least I know I would...