Very often, there are many reasons why this happens. Some students do not focus well and make mistakes here and there which reduces their overall mark. Some do not get more than 75% because they cannot solve the “thinking” questions or have trouble applying their knowledge when questions on the test differ from what they practiced at home. I would recommend practicing all questions from the textbook regardless if they are assigned by a teacher or not. Always try challenging questions from the “Extending” section of the textbook. Do more than is assigned!