I can't say how hard it is to really achieve anything objectively, but I would suggest that achieving better results does not always mean putting in more time and effort. Better results require different processes, not necessarily more difficult ones. That being said, if you're measuring difficulty in terms of how focused they were when studying, then I would say that people who score in the 99+ range generally use techniques which require a higher cognitive load knowingly or unknowingly which leads to an improvement in their learning efficiency or have greater consistency within their study and revision strategy.
I'm aiming for a 90+ ATAR, mainly because I signed a document that says I have to jump into the Yarra river if I don't, and that the principal of my school told me that you need talent to get anything above 90 and I want to prove them wrong. Currently, I'm doing practice exams (partial or full) for business and data analytics almost daily and am aiming to do one section for English per day in addition to 3 essay plans (1 per section) every day. For physics, I'm making a cheat sheet and will work out how often I can complete exams for that later. For specialist, I've got to first of all finish the coursework, then revise my weak-points, and do a few practice exams. I look over the answers of any tests / exams I do and make annotations for areas for improvement based on exemplar responses and or teacher feedback.