So by taking AP chemistry over pre AP chemistry you make the calculation of achieving at least an 85 in the course and every point above that is a bonus. You have made your grade comparable to a high A in the pre-ap class and it will boost your weighted gpa higher because of the additional credit. The difference in the weighted gpa by being not grandfathered into the classes has changed the weighted gpa from a 4.777 to a 4.72. May not seem like a lot but at the top every hundredth of a point matters.