Perhaps the naturally smart ones have weighed up the benefits of studying hard compared to the benefits of not wasting the best years of their lives for something that is really not that important, and achieved an appropriate balance of study/not study. They know what it's like to be smart, having been naturally smart all their lives, and realise that it doesn't matter whether other people think they're smart or not, what matters is that they enjoy their short time on earth, and are better off not wasting it. They also understand that the ability to study is by no means a reflection of intelligance. A complete moron with a bit of determination can go through the text books and put in a ton of effort remembering everything, handing in essays etc. This doesn't make them a smart person. I see it all the time, people who are doing well in school, thinking that makes them smart, but are all fucking stupid, lacking the ability to think logically, problem solve, work out things for themselves, understand the things they learn. All they are good at is regurgitating the things they have been spoon fed through school.
The dumb ones, on the other hand, come in two different types. Theres the ones who are stupid, always have been, don't really care, drop out pretty quickly and go get some mindless job etc, that sort of thing. The other, more relevant type, are the ones who have never been in the top classes at school (or have, but have always been beaten by the 'naturally smart' kids) and as such feel a yearning to achieve, beliving that the path to success in life is through the 'top of the class'. They don't know any better because they have never been on the other side of that barrier. So they mindlessly slave away at their study, lacking the intelligence to look at the world and ask "why am I doing this? will it help me? is it worth it?". Eventually, of course, they begin to overtake the naturally smart ones, which is a huge ego boost for them. They then achieve very well in the HSC, go for the standard "I did well in the HSC" uni courses, get a high paying job and continue to work their arses off for the rest of their lives. They continue to feel the stress and lack of free time that came with the HSC year throughout the rest of their lives because that is what they have grown to be like. They think that that is what life is like, and there is no way they can change it, or no way they would want to change it anyway because earning money and appearing smart is the sole purpose in life for these people.
? Simply because you later mention that the natural intelligence of some HSC students may be offset by their lack of perseverance / academic drive (i extrapolated this point, from the implications of your argument).