My own experience is similar to JonLaw's above. I have two full siblings and one half-sibling, and the only one who is in my intellectual neighborhood is the half.
Take one gifted girl in an anti-girl era, simmer on medium heat with a rebellious personality, add one unremarkable boy with a pinch of "don't you dare date him," shake vigorously, bring to a rolling boil, and serve while scalding hot.
With that said, if your mommy-gut is telling you that these test results don't reflect your child, in cases where the parent is not afflicted with my-children-are-a-reflection-of-me-and-therefore-must-be-the-greatest-ever-itis (and that sounds like it's most definitely not the case here), then you should go with your gut and investigate further.
