in year 10, you may have high aims, but easier said than done. sorry, but this is reality
it is fairly easy to get thrown by an exam or questions in any subject, so even though you're capable of a 90+, one exam can easily thrash you to get maybe 80 instead, even though you are a good student (e.g. the question was very specific, and you don't enough detail on it so you got thrown, or the time factor).
so even though you say I can get a 92 in BS or 43/50 in EE2 or EE1 (it is pretty hard to get in both actually, I got 42/50 for prelim EE1 and english is definitely not a low maintenance sub), that can easily change in year 11. Plus, HSC content is usually more varied and difficult, so even if you were a 90+ student in year 11, you could easily become a band 5 student in year 12 if you didn't pace yourself fast.
scaling changes every year, but I think BS has kind of a lower scaling by a little than EE1, which is a bit less than EE2. However with those marks it isn't a major significance like a drop or gain to an ATAR.