UNSW incorporate degree of rurality into their calculation. If those applicants were, say RA4 equivalent and you are RA2, that could be the difference? Correspondence from UNSW on this topic is usually quite useful/accurate. For example, the year I applied I got an email saying my combo of scores meant I had a 5% chance of offer and did I still want to attend an interview. I didn’t because I was fortunate to have three other offers by then and it would have meant flying to NSW for a 5% chance. UNSW are the only uni I recall that provide this advice, so, unfortunately it probably is accurate but you could contact them to check. If you don’t have other UAC preferences you’d be interested in adding you could always just leave it.
ETA: the lowest we had reported here to MSO last year was 56th.