While AP tests are usually harder than their corresponding subject test, the subjects tested don’t completely overlap. The biology subject tests for example tested me on subjects I never even heard of in my AP course, but for the most part they are similar in content. You could be great at calculus but that doesn’t mean you’re gonna ace Math II since there is no calculus on the test. I just wish colleges could use AP scores for admission because those tests are far more in depth and aren’t just multiple choice.