Our district is phasing out the use of the CogAT, as it apparently has a very high false negative rate, particularly at the younger ages. This could be the case for your child and could explain the discrepancy.

Have you seen how the CogAT is administered? In our case 2nd graders have the test read to them, and then have to fill in bubble sheets. Both of these are odd expereinces for an advanced child, especially one with auditory issues.