At my office (pediatrician), if you are late for a well visit and we are booked we usually ask to reschedule. If you are late for a sick visit, you are squeezed in because sick kids need to be seen- though we do advise that you may have a wait and/or be seen by another doctor.
We usually run on time, or pretty close to it, but things do come up that cause the doctors to run behind. Most often, a patient will call and book an appointment for one thing but then come in with another, or different problem. Doesn't mean the doctor shouldn't spend time with that patient for those extra issues, BUT, if the issues were mentioned at the time of scheduling, then the appointments would have been scheduled differently (or time would have been blocked in the doctor's schedule) to accommodate the extra time needed for that patient. We also tend to get a lot of parents that make an appointment for one child and then bring in 2 or 3 that also need to be seen. They're not turned away if they are sick, but since they don't have slotted appointment times, it can back things up.
We also have office hours on Saturdays and are open late into the evening (7:30, or so) in an attempt to accommodate our patients. Also, if somebody calls at 7:25 and needs to be seen, one of the doctors will usually wait for that patient to get there (depending on the reason for the visit of course).
I'm not a doctor and I am not trying to defend all doctor behavior, but I think in any type of job where you are dealing with a human element, it is not always easy or possible to predict exactly how long something is going to take. I usually give some leeway on wait times at my own doctor's appointments because of this.