I don't agree with the previous few posts. If it were true, then the averages should be roughly the same between texts, but I've found that the more complex texts generally top the rankings, so obviously the students within these cohorts aren't killing each other off. And to quote what EZ said:
Not really - either way, a good essay is a good essay.
To explain further, the score out of 10 is not a ranked mark. It doesn't matter how others write within that text; if the markers are following the criteria properly, the competition within a text should not affect one's mark. A good essay is simply a good essay. That being said though, to explain why public schools generally choose simpler texts and selective/private schools choose complex ones, it's because generally I find that simple texts can yield decent marks without too much study, while complex ones require some effort on part of the student. However, the complex ones are complex because there's things that you'll find once you put effort in, and after finding these things in preparation for the exam, often you can use them in the exam to provide an easy way to impress the examiner. Basically, complex texts often reward preparation, whereas the distinction between those that prepare for a simple text and those that don't is far more marginal and thus slackers have less of a disadvantage.
Also, complex texts are generally thematic based with various social/political messages whereas simple texts are often largely character and drama based. Given that the issues at hand with complex texts are obviously much larger, it's harder to just walk in and crap on about them. On the other hand, drama between characters is obviously much easier to rant on about having just read the book or seen the movie just once.