I also wonder if there is also a difference in focus between your university and the AP exam.  For instance my highschool allowed use of TI-89 and focused heavily on concepts and word problems.  This was similar to the AP exam, so I was top of my class in Calculus and scored a 5 on the AP exam.  However in college I struggled in post BC calculus like diff-EQ because they put a strong emphasis on differentiating and integrating by hand and banned all but the simplest calculator.