I'm in the same boat (I had a 16% difference as well). Unfortunately one of my classes didn't perform well, and essentially those who ranked lowly ended up performing the best in the exam, and those up at the top didn't perform as well as hoped. Luckily for me, it was an extension subject so the scaling helped to relieve the affected mark.
But really, all those who have performed well in the HSC but with a low assessment mark, did deserve it in a sense. You didn't do well at school, or even if you did, your rank wasn't as good, so you only need a few other people to bomb out in the HSC to affect you.