You need to distinguish between absolute observations (child is hyperactive) and relative observations (child was more hyperactive today than yesterday). The meta-analysis cited above uses relative observations. That's why I wrote,

This isn't saying that parents reported more hyperactivity than professionals. It's saying that, across 15 double-blind placebo experiments, the behavior observed by parents had a strong correlation with whether the child received the test substance or the placebo, over four times as strong as that measured by professionals.

Also, this was across 15 studies, not one study.