Ok, listen.
UAI is determined by two factors basically: your internal mark and your HSC mark where your internal mark is adjusted according to the list of HSC marks from your school's cohort.
I gave your estimate based on your internal mark/rank which counts for 50%. Whether you get higher/lower from that entirely depends on the other 50%. But there are so many things coming into it. Your school's performance comes in and your own hsc results come in. I have discovered that doing relatively better in HSC is actually better than doing well in internal. Only explanation for your significantly poorer/higher UAI than the estimate is that you either did extremely shit or extremely well and that's highly possible.
And you also have to consider that UAI estimate is only an estimate. Even the calculators that use YOUR OWN HSC results are not accurate. How can mere human beings with NO OFFICIAL HSC results whatsover give you an accurate one?