The notion that your school certificate results are an indication of your HSC results is utterly ridiculous; people who perform poorly in the school cert. can pull their socks up (metaphorically speaking) and do remarkarbly well in the HSC. Likewise, some people who perform well in the school cert. can bomb out in the HSC because they burn out, or chose the wrong subjects. Furthermore, its also worth noting that the School cert. has a pre-structured syllabus, which students must adher to; whereas in the HSC students are able to choose their subjects, and thus focus on their strengths and interests.
However, there is some truth in saying that students who perform well in the School cert, generally perform well in the HSC: there is no mystery to why however, as this is merely due to the fact that students who are bright and/or conscientious students in year 10 will continue to be as they embark on their senior years. Likewise, those who are not naturally bright and/or are not committed in year 10 may continue with this attitude as they complete their senior years. Thus, it is these facts that would explain the apparent link between a students school cerificate results and HSC results.