OP asks an interesting question.
I did my HSC in 2012.
And yeah, I got a shitty ATAR in the 50s. During that time, I've had to reexamine: where did I go wrong? How do I take responsibility? Sure, I debate about whether the HSC is fair, but I don't want to come across as a sore loser or diminish other students achievements. Yes, I had a medical condition that was extremely bad. But I don't want to use my traumas so I can be viewed in a different light. Pretty much? Just cause I was a sob story in 2012, doesn't mean I'm going to be one in 2018.
The HSC does have an emphasis on memory. I remember getting back English essays- and the feedback was always 'not enough quotes / evidence'. Nothing about my critical thinking, understanding of the texts, or whether or not my ideas were any good- but how much I could rote learn. Modern History (which I'm now pursuing as a major, and doing quite well in) was just information and date after each other. When really- the academic field of history is all about understanding the past, not just how much data you can recall. When people say HSC subjects are nothing like the ones at university level- this is what they mean.
I did study during the HSC. Not as much as other students, but still a decent amount. Teachers and even students would often take me aside and say that I was saying really interesting and unique things in class. That did well for my confidence, but it never translated into marks. Maybe I had neither 'hard work' or 'talent', and I believe sometimes those elements can take years to fully develop. I can get credits and distinctions at university, no problem.
My sister got an ATAR of 98.50. She's very good at remembering data (talent), took the HSC really seriously (hard work). But she knew that she was 'playing a game'. Sometimes that's what the HSC is. A game. To me, that sickened and revolted me. That I was forced to see my friends who I had spent 6 years developing a relationship with- as potential backstabbers. Then, once after trials were finished they acted as if we were best friends for life. Yeah, lots of fakes around. Everyone in 2012 was just so fake and pathetic. All for a fucking rank and number.
So yeah, I take responsibility for my ATAR. I played the game and lost. The HSC is a game. It'll make you angry and upset, years after its over. Some of it will be directed towards yourself. You'll go on Whirlpool, and you'll be told your 80-something ATAR is 'nothing special' and have someone yell at you for being lazy. But screw them. I work my hardest now, I'm proud of what I've accomplished post HSC.
So natural talent or hard work? Maybe both. It's also luck and memory.