This is a fair point.73/100 would put you above 90 definitely.
However, remember that your own marking is not an accurate representation of the HSC marking.
Also, if I were you, I'd be revisiting the questions you couldn't do for the HSC. This will ensure you have the best chance to tackle the really hard questions that come up, rather than simply being stuck with a lot of marks and a lot of time nearing the end of a test
Given the amount of questions in a 4u paper that are prove questions.
Is it possible that someone could be marking themselves 80 and actually be getting 60?