GPA mongers lose out to students willing to take on tougher classes

My youngest daughter, now graduated from high school as of last night, concluded her senior-year AP stats class with a study on some factors that may predict performance on ACT testing of her college-bound classmates. She and her study-mate got data from 38 students on their GPA, ACT and the number of AP classes taken. The teacher did not give much direction on what to make of this information, so when my daughter asked me to help, I steered her to a modeling of ACT as a function of the GPA and AP. This is pictured here in an output from Design-Expert® software. What I find interesting is that after accounting for the impact of the number of AP classes (highly significant, p=0.0003), the students' GPA makes essentially no difference (p=0.4596).

It seems to me that this provides an insight on the impact of being motivated to learn. At our high school, those aiming for high GPA do better by NOT taking the relatively difficult college-level AP classes. Those who do load up on APs are willing to chance a lowering of their GPA in return for an enriched curriculum. Is it any wonder that these are the ones who do test better on ACT?