Sure, in exceptional cases investigation is probably necessary. But there are still more marginal cases where one exam gets worse results than another. >Your a

Sure, in exceptional cases investigation is probably necessary. But there are still more marginal cases where one exam gets worse results than another. >Your answer has not convinced me that giving an A to a student who got 55% correct on a test displays a command of the subject simply because everyone else got 30% correct. The point is that it's just as unconvincing to say the student that got 55% is comparable to another student that got 55% in an exam where that was the lowest score. Exams are not perfectly comparable measures of absolute "mastery", so we have to use statistics to try and derive as much information as we can from their results. If students in one cohort consistently perform worse, it's probably their fault, but if students in one exam perform worse, it's more likely the examiner's fault.