Back in '83 I got an estimate of 35 and got 70 in the exam, my final mark ended up as 46. It was a different system then so does this count ?
Mind you I dont blame the teacher overly even though he hated me. I guess he didnt like me because I only did tests and none of the other assessments - hence a poor estimate.
BTW back then 70 was good enough for the top 10% and 46 was 41-50% range.
There are at several possible reasons for a very different estimate. You didnt work too hard during the year and "earned" a low estimate though you studied a lot for the HSC at the very end. Your teacher under/overestimated you. You perform better/worse on tests than other assessments.
From my experience the vast majority of the time it is the first one - the student didnt work at his best during the year but studied like crazy just before the HSC.