My classmates are some of the smartest people I know. But it wasn't until our first couple exams until I realized why people often say that exams are nothing more than your ability of test performance. Taking an exam is a performance. There is no guarantee that the exam will be representative of everything you learned. There can only be good faith in the fairness of the instructor.
For example, if an exam only tests you on three of the pre-determined questions, it obviously does not test the breadth of your knowledge in your ability to answer all twenty long-answer questions that the instructor gave you beforehand.
And then you realize that this is life. In gymnastics, people only see the couple-minute routine on the balance beam. In Hearthstone or other card game, your hand combined with experience only gives you a higher likelihood of winning, not a close guarantee. When you do a medical training simulation, you reduce your error rate of performing that life-saving maneuver.
By obtaining mastery in a subject, sport, or other skill, we effectively increase our probabilities of success. That means that there is still the possibility of failure, of not knowing what I'm supposed to know at that particular moment, despite how well-trained I might be. Knowing that even the best have moments in life where they fail, even though marginal, helps me to take pressure off from myself, whether in a test or another everyday circumstance.