This is based on the average score. This average score is not simply to add up your grades in each course and divide them by the number of subjects, but also related to credits and grade points. Let me make an analogy. You have a compulsory course with four credits, and you got 80 points in this course, so your GPA is 12.0. Another elective course has two credits, and you got 70 points, so your GPA is 4.0. Finally, your GPA is (12.0+4.0)/4+2 = 2. In short, subjects with more credits get heavier scores. If you don't understand, please keep asking me.