So what would you do if your kid was in the top 2% in one subject (eg English), but in the bottom 2% in another (Maths)?
Would you expend your energy on the weakest subject, trying to raise the outcome? Or on the strongest, where you could get him to uni level aged 14 or so with a bit of effort? The government's policy is to spend more money and resources on raising low-scorer performances, but what about different abilities within the same child? Or within the same family? Do you spend your energy on your chess prodigy or your underachiever?