I think what school you go to plays a part, as well as your cohort.
I did a fair bit of study, but not in the form of past papers. I did maybe 8 past papers for Ext 1 Maths, and 1 for physics (Both band 6's/E4), but I did lots of revision, wrote practice essays, made summaries and wrote out all formulas required. I got 96.00.
I'm not going to lie, I thought I was going to get a little higher, but I am definitely happy as it's more than what I need to get into the course I want to do.
In short, I reckon I worked really hard, and got a really solid mark in the end. Although, it is also my opinion that this year, certain subjects were marked harshly and ATAR's are lower than expected across the board [at least with people at my school].