My husband maintained a perfect 4.0 GPA all the way through his PhD (Physics B.S., Physics M.S. Physics PhD). He never cracked a book. Seriously. None of it was ever difficult for him. The prof I took Classical Mechanics with was also my husband's instructor for the course (though Hubby took it years before me). The prof shared with me this anecdote about my husband:
Hubby always blew the curve on exams in the class. When the class average/mean (minus my husband's score) on an exam was around 60%, Hubby would still get a 98%. So, for the final exam, Prof decided to make the test as hard as he could...like, so hard even grad students would have difficulty with it. The Prof's stated goal (as he later recounted to me) was to see if it were possible for my husband to get a B on anything. So, Prof gives the final; the class average was around 45%; Hubby scored an 90%. Hubby wins.
In short, he's a one-in-3-billion FREAK OF NATURE.
To his credit, Hubby did take AP Calc and AP Physics in high school with very, very good instructors (and he got 5's on the AP exams, natch), so he was very well-prepared for undergraduate physics. He says so much of the first couple years of undergrad physics is a re-hash of the AP curriculum that it was just too easy. In a sense, the AP background allowed him to jump ahead of his classmates, and he just stayed there. When everyone else was cramming for tests, he was in his dorm room playing video games.
That said, Hubby is a brilliant mathematician, but he's not especially creative. He can find the solution to any math or physics problem you put in front of him, but he's not all that good at finding novel research ideas to pursue. He has a good career, but it's unlikely he'll ever win a Nobel, for example, since he's not the creative type of clever that that kind of ground-breaking work entails.