The difference can be mitigated by getting as many school marks as possible. If 1st in on 85% and 2nd is on 84%, that is a better scenario than if 1st is on 85% and 2nd is on 75%. In the first scenario, your assessment marks will be adjusted to reflect that small difference. In the second scenario, even though you're 2nd, your marks will be adjusted to reflect that large gap.
And even in these 2 scenarios, this can be mitigated by achieving as many HSC exam marks as possible for everyone in the cohort. Once all your assessments are finished, you should really help everyone to gain as many marks as possible as the average of the exam marks is a factor when adjusting assessment marks. The higher the average, the better the assessment marks will be adjusted to. So since the average is affected by everyone, it makes sense to help each other.