Currently VCE has been in for nearly 20 years and DECD will be looking at a change in the system sometime soon. Most people do 13 years of primary +secondary education and it comes down largely to a few exams, to a few days, and a few short moments.
You could seriously just have a terrible day in an exam room and ruin your prospects.
American universities take into account greater performance over high school in general, and personality and characteristics as well as co-curricular involvement, along with performance in final year. They have the luxury of this as they have much greater demand for university placements. Also there is an arguably greater equality shown in Education in America, unlike in Victoria, where only 2 public schools (and also select entry) consistently place in the top 10 based on VCE results.