My understanding is that a notional band 6 is the level of performance at which it is expected that you would have performed, had you taken Mathematics Advanced. This applies to students who take Mathematics Extension 2, such as yourself. Because you took Mathematics Extension 2, you were not required to sit the Mathematics Advanced HSC exam. Even if you accelerated Mathematics Advanced prior to taking Mathematics Extension 2, taking Mathematics Extension 2 overrides your result in Mathematics Advanced, meaning that Mathematics Advanced would no longer count towards your ATAR. Because in such a case, you would not be considered to have a particular result in Mathematics Advanced, you will also be assumed to be able to perform at band 6 level in Mathematics Advanced, had you taken it.
What is interesting is that some schools celebrate notional band 6's, i.e. they solely celebrate the fact that some of their students took Mathematics Extension 2 in the first place, regardless of whether they achieved a band E4. An example of this is Frensham School and its 2022 year 12 cohort. In addition to celebrating high-achieving students, they also list students who took Mathematics Extension 2, mentioning that these students achieved a notional band 6, despite the fact that those students did not achieve a band E4 in Mathematics Extension 2.
I hope this helps!
