Basically what Seventhroot is trying to say is that BOS only looks at the internal rankings and the relative gap between each of the ranks. The actual marks of your HSC exam is determined by the external exam, in which the highest external exam mark will be used as the highest internal assessment mark for your cohort (converse applies for the lowest ranking person and mark). You keep your own external exam mark and it is averaged out with your internal assessment mark to give the final exam mark (the one you see on the website), obviously there is alignment yada yada but I don't know where that comes in exactly. This is why people in ruse, although having "low" internal marks, are still able to achieve band 6's as the overall cohort does well in the external HSC exam.
http://www.youtube.com/watch?v=TW2jEyjEJiM This video probably explains it better.
P.S. if the information in this post is incorrect, ignore it completely lol....