That's correct.
No, a Band 6 does not mean you're in the top 10% of a subject. A performance band, i.e. Band 6, Band 5 etc., are
measures against a defined standard. There is no quota or capping on how many students can achieve a Band 6.
An ATAR of 90 means you're in the top 10% of your Year 7 cohort, not your Year 12 cohort. In fact, an ATAR of 90 means you're in the top 16.5% of your Year 12 cohort (2016 data).
But you need to understand that the ATAR is calculated using scaled marks, not HSC marks. BOSTES gives UAC two marks - your raw HSC exam mark and your raw moderated assessment mark. These marks, which you won't receive unless you pay for it, are scaled. UAC performs scaling to ensure one set of subjects can be fairly compared to another set of subjects. This has no correlation to Band 6s.