As a Ruse student, something a lot of people overlook is how being top of the state for 20 years gives us multiple unfair advantages:
As many have posted, we (usually) get the best students from the get go due to our popularity in Selective, but also we steal everyone else's best students in the form of transfers from other schools. By Year 11/12, a quarter of the cohort is made up of students who transferred to Ruse after Year 7 from other schools. Obviously, this gives us an advantage in our HSC success rate: the percentage of band 6 results out of total entries, which is used to calculate HSC school ranks. Taking students pretty much guaranteed to get band 6 in all their subjects is obviously and advantage: Ruse had a success rate of 73% last year compared to Baulko's 62%
Also, students, teachers, and parents simply expect Ruse to come first every year. Ever since Ruse started dominating the rankings, the whole school is geared towards maintaining that dominance, competing with our predecessors to get the most .95s and state ranks. There's definitely a culture of confidence, sometimes even arrogance, but it causes students to be dissatisfied with anything but the best. Year 12 students only give their ATARs as a ".3" or ".8", a "point 95", there's always this expectation that you'll break 99. At schools like Baulkham Hills, people aren't shocked when the school doesn't come second, because it doesn't feel guaranteed. Hence people don't tend to try as hard.
Our long 'reign' also means there's a well-established network for study. Peer learning is definitely a thing, and maybe even more importantly, there's a lot of information on various tutors. In maths class, you hear people talk about Kurt, Mr Guan and Dr Du (various maths tutors), and ex-students and parents start up their own coaching places. It's easier for us find classes because of this network of experience, but also the tutors are more willing to take Ruse students, as it generally adds to their prestige and improves their results. People argue that tutoring isn't a big factor in your ATAR, but 90% of students at Ruse take classes outside of school, and most of those do tutoring for 3 or more subjects. That's where most of us find the extra edge to push us to towards the top marks. School teachers often don't teach everything well! Good tutors can make a ton of difference.
There are probably a ton of other reasons, but these are the ones that often are invisible from an outside perspective. Obviously these are just my theories speaking from experience at the school. If you disagree I'm super interested in your ideas! If you have any questions please shoot.
EDIT: To clarify on my tutoring point: you can definitely achieve excellent marks without tutoring, but it's much more difficult. You need to be smart, hard working, and have good self-study skills. Ruse students are usually one or two of those things; tutoring makes marks like 99.95 possible without being a genius.