I think you underestimate some private schools. At least at mine, high level APs would have only 10ish specially approved students with a professor who'd taught the class forever. Our calc BC professor had supposedly only had 3 non-5s in his decade or two of teaching the course. Besides that, if a professor suspected you wouldn't pass, you would just be asked to not take the exam. Don't think I ever heard of someone actually failing one.