For in school assessments, its impossible to make sure that all schools are giving their students tasks of the same difficulty. If they reported your assessment mark as the mark you got at school, schools who have their students easy tasks would be advantaged over those with harder tasks. So what UAC does is line up the schools assessment marks against its HSC exam marks - which means that if you get 10% for you in school mark but top your class, but then get 90 in the HSC and top your class, your assessment mark will come out as 90.
I hope that made sense...I was in an economics exam for three hours today, so you'll have to excuse my lack of explaining things ability.