Say, for example, you get a raw assessment mark of 80. Then you go into HSC and get a raw exam mark of 70. The BoS align this according to their standards (Step 4 in
http://www.boardofstudies.nsw.edu.au/hsc-results/determining-achievement.html), so they might align to a 75 for instance. Then the BoS uses this mark to 'moderate' your assessment mark as I described earlier - but there's only one person in your cohort, which means that you will get the exact same moderated assessment mark as your aligned exam mark - so you'll get a 75 for both. These two marks are the ones you receive, and your final HSC mark is the average of the two.