ALL SC and HSC external exams are put through an aligning process which basically converts the range of exam marks from 1 - 100 to a reported range of 50 - 100 with about 1 - 2% of the state in each course receiving a mark less than 50.
The SC grades are determined totally by your school and have absolutely NO relationship to the external exam marks.
I have taught a student we gave an E to at school (and rightly so based on results) but who was able to gain a Band 5 on the external exam due to spending the weekend before the exam reading their textbook thanks to a new found love of History. The two results were accurate reflections of what they were separately reporting - the grade reports a year's effort whereas the Band reports a single exam performance on a given day. Think of your school reports - do you have (as my school has) separate marks for assessment, exam, class work? Well the Grades are the Assessment and the Class work whereas the Bands are simply the one off exam.
As a teacher I am far more interested in the Grade awarded in Year 10 then one exam performance and when students enter Year 11 I ask specifically for their Grades not their Bands.