I think the most important reason students pick up their game in Year 11/12 is because of the subjects they pick.
e.g. There are plenty of cases of students studying all the mandatory subjects (English, Maths, Science, Geography, History, PE) and getting extremely average/un-extraordinary reports, but once they choose the subjects they're actually keen on, their marks improve. But it all comes down to work ethic, motivation and enthusiasm I reckon.
Honestly, for me, the only reason I'm motivated to study in Year 12 is because, if I don't get a decent ATAR (in my mother's eyes) she's going to give me sh~t until the end of my life (like she's been doing to my brother) and she'll ship me off to another state/city to study Law.
In the case of my friend, s/he got above average marks for all of their subjects up until Year 10, did relatively well in Year 11, and now their ranks continuously drop because they chose subjects that "scaled well" and Asians were expected to do (you can guess). Now the course they figured they want to do has nothing to do with these subjects so her/his interest levels are dropping, dropping, dropping... just like their ranks.