The reason grades cannot be more accurately predicted is that the exams (of which there are, of course, no past papers yet with which to compare) is that the exams are supposed to be harder.
My understanding is that previously over 90% would pretty much be an A, over 80 % an A and over 70% a B etc. Therefore if 30% of the cohort scored over 90% they'd get As.
However as the exams are likely to score lower percentage grades it will be strictly done on a curve and a certain percentage given 9s (very few), then the next percentage 8s etc. So if you factor in clever cohorts someone who may have got an A* previously may not get an 8 if they do not fall within the percentage available for 8/9. So a proper curve system (like when I did my O levels).
The other things goes to the percentage being achieved in the exams. If the exam is harder then the spread of scores is likely to have a wider range thus differentiating between the top of high end (9) and the bottom of high end (8). Until there are some actual exams with grade boundaries set and then a number of these it will be hard to say what is a likely 9 or 8 or 7 (or 6/5/4). There will also be distortions where if one school year are not quite as clever you may expect say (pre example) 68% got a 9 last year. However if this school year is cleverer then it may be that you need to score 73% to get the 9. (All hypothetical scores until there has been some actual sampling).
When schools get the results at the moment (under the A* - G system) do they also know the percentage scores the child gets. So would they know whether the child got over 96% or between 90-95%? If so would this be an indicators as to whether they are potentially 9s rather than "just" 8s.
If so then they would then be able to use this date to guesstimate whether a piece of work may fall within a numerical scale in that they could compare to previous students' work for comparisons to high As to low As.
Wow _ I hope that makes sense (it does in my head) but I standed to be slated corrected!