When you sit your HSC exams you will usually get some mark between 0 and 100.
But the thing is, each year the exams vary in difficulty so an 87 in 2017 may not be the same as an 87 in 2018. And obviously, an 87 in Maths Ext. 2 is very different to an 87 in Maths General 2.
So what NESA does is...