I had a similar experience in college. In the classes I did best in, I studied for consistently throughout the week, working out problems and reading the book on my own. Then, when test time came, I still studied, but I largely understood the material already.

After a calculus class ended for the semester, I had the professor tell me that I was the only student that understood a certain basic concept of the class. I don't attribute that to me having a masterful intelligence, just that I happened to be diligent in the class and figured out a system based on seeking to really understand the material instead of just pass the tests.

The classes where I heavily procrastinated, I had more trouble with, or at least much more stress. There was one class where I procrastinated and then speed-read 300-400 pages in the textbook the days before the midterm, and aced the midterm...but that was really a freak exception to the rule. It probably did me a disservice, thinking I could do the same for other classes and still succeed.