I think it’s a combination, really.
Theoretically, if you know the material thoroughly enough, you should be able to succeed in an exam. But we know that’s not always true in practice. The way questions are worded can sometimes be confusing or misleading, causing students who otherwise deeply know the material to appear as if they don’t. It’s not a perfect system. So working harder should help, but it’s never a guarantee. Such is the reality of exams. Standardized exams can be the worst. They’re designed differently than your everyday class exams, so they’re usually much more difficult. Thus, if you do well on your in-class exams but not on the AP, so long as you’ve been keeping up reasonably well on the material, the culprit is likely how you interpreted and reacted to that specific exam format. Among others.
When I took AP Bio I had a 98 average all year. I aced the exams, high scores usually. I loved studying for that class more than anything. It was fun. I was clearly spending my time studying. But come the AP I didn’t get a 5. I talked to other students who took AP Bio elsewhere and those who had the hardest AP Bio classes typically received 4s and 5s on the AP. That’s because the professor pushed them further than they needed to go for the AP so that they’d be more than prepared. It might have hurt GPAs a bit, but as long as you get into the college you want, high school grades matter 0% after graduation. So in that sense, AP credit is worth more than getting the perfect grade in high school, in my opinion, at least. So another disparity can be between the relative difficulty levels of your teacher’s exams and the standardized ones. They don’t always match.
The good news is that within the context of your own class, the difficulty level of the exams should reflect the rest of the coursework. Not always, but they ought to. Be sure to ask upperclassmen or consult Rate My Professor (not the best source but it’s something) to get an idea of how difficult the exams are in comparison to the rest of the coursework. Even if they don’t match, at least if you know that the exam is going to be harder than the class in general, you can better prepare for it.
The truth is, some people are not good test takers. Some people are excellent test takers. Grades are important, but grades are not everything. A great test taker might get a 100 on an exam she studied properly for while someone studying the same way might only get a 95 because of weaker test taking skills. Sure, the first person technically got a higher grade and “did better” on the exam, but who is to say that both of them don’t know the material equally as well, just the second was more prone to mistakes because of poor test taking skills? Grades do not directly translate to success in the real world, either. So while all of this is important to consider, don’t forget that applying what you learned to a future job is ultimately more important, and that might be something you’re better at.
I would recommend you pay close attention to finding study tips that really work for you and learning how to better take tests, if you can. Working harder can help, but only if you’re also working smart. Rereading a textbook 7 times is working “harder” than taking notes on the book once but it’s not a better study method by any means.