Welcome NT! So sorry no one seems to have seen this earlier.
We do not have a child with this exact profile, but several other posters here do. And yes, this is pretty common.
If diagnosis is of specific value in your situation, have Ehlers-Danlos or other ligament laxity syndromes been discussed? Those are often overlapping with the symptoms you've described. In any case, good to hear that you have access to therapies with or without a diagnosis.
As to why? Many schools view significant impact on function entirely through the lens of grade-level expectations, so a student whose intellectual potential is well above average is still not viewed as underperforming, because they are keeping up with ... grade-level expectations. In some regions, this is actually codified in regulations regarding educational disabilities.
