These exams often work by having the examiners ask questions until one reaches the boundary of the student's boundary, partly to see where that boundary is and

These exams often work by having the examiners ask questions until one reaches the boundary of the student's boundary, partly to see where that boundary is and partly to see how they handle things they do not know. Ideally, anyway. The oral examination format can be very intimidating, and committees often don't care to (or don't know how) to put students at ease. So how well performance correlates with actual ability really varies.