This is crap!
If i got 100% in the yearly trial mathematics examination, does that mean that it would come out as say 95% because i go to a crap school????
No.
It means that if you go to a school which sent to the BOS a top mark of 90 and another school sends in a top mark of 60 but both schools top exam mark is 70 then the top students at both schools will be 70. However if the top mark at both schools on the external exam was 100 then that would be the top mark for both schools. The school's top exam mark becomes the school's top assessment mark (unless there are equal firsts in which case they average the top two or three marks depending on how many students were tied).
Your trial mark only contributes part of the mark sent to the BOS and that percentage will differ from school to school.
To get some form of parity between schools with their different assessment tasks and weightings then the BOS has to come up with a way of equating those marks.
They do this by using the range of marks earnt by the cohort in the HSC exam. It is the range of marks from the exam that allows the BOS to put a range of marks on the school's assessment marks. Some schools set much harder tasks than others so the BOS wants to be fair.
The moderation means that if the school sends in a range of marks from 30 - 90 and the exam marks are 50 - 95 then the moderated assessment mark's range will also be 50 - 95. The total number of marks for the school will also be the same so if the school's total exam marks was 1499 then 1499 marks will be allocated to the school for the moderated assessment marks. Finally the top and bottom moderated assessment marks are set by the exam (but the other assessment marks won't NOT necessarily match any other students' exam marks).