See also https://btv-open.dreamhosters.com/2016/08/16/roller-coaster-academic-competition/

I often feel a primary school kid improves his benchmark performance

* either because he put in effort, perhaps due to parents’ help -OR-
* she has extraordinary abilities

If a kid is reasonably bright but do not put in the effort, then logically she’s likely out-shined by other kids. That would be my explanation of my son’s situation.