Again, it's all about the distribution of students. Most kids at MHS/MacRob are obviously "smart", because they've gotten into the selective school. The Jewish schools are 100% Jewish (lol), and from what I've heard Jewish families are very focused on making sure their kids do well academically, hence the high scores all-round. If you had a school composed entirely of a similar ethnicity, my instincts would tell me it would also do pretty well (a school of 100% Asians/Indians, for instance...).
In comparison, even the most elite private schools can't compete simply because not 100% of their student population will be the high achieving sorts found in the aforementioned schools, as there will always be kids who simply don't want to learn or who aren't as capable academically as kids who are "smart" or who have families encouraging them to do well all the time. Of course, I'm not saying that EVERYONE at selective/Jewish schools is "zomgacademicprolah", but I'd say the difference is enough to give them a significant advantage in the rankings.