Drop a grade in gradebooks - Question

Here is a dilemma faced by our Physics teacher. Is there a solution of which I am not aware?

"For my AP Physics class, I always allow students to drop a certain number of homework assignments each semester, and in the grade calculation, you can specify the number of dropped assignments for each category. However, when I was entering homework corrections in the gradebook, I noticed a strange artifact. When I entered a homework correction for a dropped assignment, the student's overall grade actually dropped when I entered the improved homework score. Do you know how Blackbaud chooses which assignment to drop? They obviously don't choose in a way that maximizes the student's grade. If not, I will have to manually exempt homework assignments to make sure the student's grade is always maximized.    The problem is exactly here:  Blackbaud chooses to drop a 4/5 assignment instead of dropping an 8/10 assignment. They both have the same percent score, but dropping the 8/10 helps the student's grade more than dropping the 4/5 assignment does. The reason is because I use a weighted average, and dropping the larger amount of points reduces the overall point total more, which helps the student's grade more. Ironically, if their average was less than 80%, it would be more helpful to drop the 4/5 grade. I guess it is a tricky algorithm."