Well, I'm no expert, but...
If she is a "hider" who values fitting in more than achieving, her achievement scores might not live up to her IQ scores. Similarly, if she has been underchallenged for a while, doing busy work but not learning anything above grade level, she might not achieve to her potential. Gifted kids who aren't highly motivated to learn on their own may only learn what they are taught. If that's strictly grade-level material, that child may not learn things above grade-level.
A child who is not used to being challenged can feel very frightened to suddenly not know all the answers. It's like having the rug pulled out from under them. They may choose to remain underchallenged rather than to risk being wrong. It's downright earth-shaking for some of them, rocking their reality.
Some learning disabilities can also push achievement scores down.
None of this may apply to your child. I have no idea, of course, because I don't know your daughter, so take this with a grain of salt! But when I see a 99.9%ile on an IQ test that isn't matched by achievement, I tend to ask questions at least...
