InteGrand is correct. Just passing all of your exams would not mean an ATAR of 70. For example, one year, 50 could scale very well whereas in another year, it could scale very badly. You must also consider the difficulty of the exam, how well the Year 12 cohort went overall etc. Simply passing will not guarantee you the average ATAR. You must also remember that your ATAR is a rank, not a mark. It does not work like the way you are thinking.