1) Most schools want their students to do well
2) Most students do well when they're prepared for the HSC
3) Most students are prepared for the HSC when they've done HSC style questions under exam conditions
Following this logic, there's no better way to prepare a student than to put in likely questions in their trials, for example say... last year in chemistry, 6 marks were devoted simply to esterification. This alone makes it unlikely that more than 3-4 marks (MAX) would be allocated to it in this year's HSC. Correspondingly, teachers at my school did not bother focusing on this significantly nor did they make it a significant mark allocation in our trials.
Experienced teachers can probably predict whats coming in our exams. We probably can too, if we compared every aspect of the syllabus to papers for the last 20 years (but the time it'll take to do that will probably be better spent studying the full syllabus haha). Catholic schools are reknowned for setting good papers, and the teachers predicted well what could appear in this year's HSC.
Another thing is that its much more likely for an ancient history exam to be similar to the HSC exam than say a science e.g. chemistry or biology, simply because as a history the syllabus has fewer 'dotpoints' and more depth is required per dot point because a significant proportion of marks are 'essay-style' (as opposed to like science where there are only around 3 questions above 6 marks and everything else is short answer). A limited syllabus means that questions can generally encompass smaller areas-- the marks then are not so much for knowing the topic, but for knowing it well and in detail.
tl;dr (how did I just write all that) I think it was a coincidence + smart teachers
