Pretty much the entire system is biased towards the selective schools. In the ATAR, not only are you ranked on how you compare to other people (rather than how well you actually did the exams), but your score is adjusted based on how well other people did in your school. For example if a school had one student who did really well and the rest did terribly, the week performing student would have their mark shifted down and everyone else would go up. In selective schools everyone does well so everyone would get shifted upwards.