The school rank and amount of people in your cohort indicate how accurate the mark will be. Its like saying how someone 1st in JR in chem and someone 1st in some rank 400 school in chem is most likely not equal. There is a small chance that the guy in the rank 400 school gets 99 in externals and stateranks but you have to also consider how unlikely this is. So basically the smaller the cohort, the larger the range of marks you will receive. ATAR calculators are not reliable as of this stage due to how heavy yearlies weigh and how heavy externals weigh too, people tilt towards the end, and alot of people do.