a high diploma is a general education type thing, which gives you lots more latitude and variety of classes. this gives ten option of choosing "easy" classes besides the required classes of course.
Generally because of that they also require to see your transcript which specifies which courses you took. For instance to get into a science program at University I didn't just have to have a good GPA I had to have taken at least 4 of the 6 science courses offered in highschool as well as two of the hardest math courses.