Others may know more but I believe they used to print all the passports for that day in the morning before the first appointment, but there were too many "problems" (not entirely sure what) so it was decided to wait until each applicant actually turned up
Maybe people forgot to turn up, or suddenly couldn't make it, and then found their current passport had already been cancelled.
It could be that they don't cancel the old passport until the appointment, and then you have to wait 4 hours for the new one to print and be registered in the system(s).
I'm guessing.