You can't cover it all in a summer, you can only think you've covered it all in a summer. In reality, you have only a cursory overview of most topics so you fail when you go to college.

I've seen this firsthand. Kids who think they're gifted and talented, and coast with B's through high school. Then college Calculus hits and they realize they don't actually know math or how to reason about it - they only know the surface level of the pieces and how to apply them in a test.