...*sigh*... ok i'll try to explain...
The UAC's scaling of different subjects uses a complex algorithm based on a simple premise: some subjects are more difficult than others. How do they try to gauge this?
Say I got a mark for English that put me in the top 10% (90th percentile) of the English candidature, but a mark for Biology that put me in the top 70% of the Biology candidature (30th percentile). Say this pattern was repeated over a large number of people who did both of these subjects - where people tended to be have higher percentiles in the English cohort than in the Biology cohort. This makes it appear as if the Biology cohort is "smarter" than the English cohort, because there are lots of people like me who are in the top 10% for English but only in the top 70% for Biology. Consequently, Biology as a subject would scale better than English.
This is essentially what the UAC does, comparing the percentiles achieved by students in a multitude of different subjects. So yes, it does work on the premise that some subjects are more difficult to perform well in than others. However it is not driven by value judgements, as some in this thread have suggested - it is based solely on the performance of the HSC candidature in any given year. There is no reason why the maths and science subjects could not be the lowest scaling subjects next year, if that is how the candidature performs.
