These estimates assume you perform inline with your school rank (which is quite a big assumption).
If you are someone (like me) that tends to do well in exams (but are crap at internal assessments- projects/speeches etc) then you will likely get a higher ATAR than these predictions.
When I did my HSC I boosted my ATAR (or UAI back then) by 6 points in my final exams... If I performed in line with school ranks I would have got 92, but ended up getting 98.
In short- dont read to much into these estimates and make sure you do well in these final external exams.