I do process improvement work in hospitals, and this is something I'm just starting to get my hands on.
Essentially, doctors don't ever want the possibility that they might one time by chance have to wait one minute for a patient to be ready. It's not that they're behind on their procedures - they're likely starting exactly when they were planning.
We've found out that one doctor has his patients come in THREE HOURS before the scheduled procedure. Other doctors are having them come in two hours early. It takes between 45 mins and an hour for registration/pre-op nurses/doc/anesthesiologist to do their thing before the procedure. Patients then sit for an hour or two for their procedure to start.
It's horrible, and I'm sad to hear it's happening elsewhere.