Usually that is the case to achieve a UAI like that. Usually schools like James Ruse have students ranked 30/60, but are still getting high assessment marks, whereas at an average school, they may be significantly lower. The reason why ranking is more important is due the fact that some schools may mark harder/easier than others. At an average school, it is best to be in the top ten for most/all of your subjects in order to achieve a high UAI. On the contrary, at a "better" school, it is not as crucial to be in the top ten.
When you ask "do you need at band 6 for most/all of your subjects", do you mean your actual HSC marks or school marks? I have a friend who got 98.30 for his UAI and did 4U maths, adv english, chemistry, physics and biology, and I think he only got a band 6 for 4U and 3U maths. He was ranked first for every subject except English, in which he was ranked 8th. He went to an average school.
I hope that helps.