It's not that they're triple booking, it's that everyone schedules an appointment but doesn't say what they're actually needing the Doctor for. So the staff schedule according to what a patient says they are there for, which should take say 15 minutes for what the patient says they need, but after the patient is actually there, they tell the Doctor about 3 other things they never mentioned to the staff ahead. This is what takes up more time than scheduled. Then there's the people that are late at the beginning of the day. First appointment of the day at 8am but they show up at 8:15 for a half'-hour appointment, and the Doctor still sees them, because their next appointment for 8:30 obviously isn't there yet, but that appointment now also goes long.

This then compounds through the day, helped only by pre-planned empty sections of the schedule normally for emergency appointments, people cancelling or not showing up, and people that actually are there for what they said they are getting in and out and taking up less time than planned.