Ds6 took the WIAT, not the WJ, but his achievement scores were higher than his IQ scores.
That said, he took the WPPSI at 5y5m and was really too old for it (though I don't know by how much -- if anyone can find the WPPSI technical report, let me know! I'm jealous of all the WISC information!), so his IQ score was likely at least somewhat of an underestimate.
And since he was 5, the achievement scores may have been somewhat inflated.
That said, his Mathematics composite and his Word Reading (we didn't get a Language composite) score were quite a bit higher than his IQ scores. Spelling was quite a bit lower than IQ, Numerical Operations a bit below IQ. Math Reasoning was about the same.
This is what that segment reads in the report:
Ability-Achievement Discrepancy Analysis Predicted Method
Ds5's scores on the WIAT-II were compared to the levels of achievement predicted for a student with his general cognitive ability, as indicated by his Full Scale IQ score of 148 on the WPPSI-III administered 11/13/2007. Significant differences between actual and predicted achievement scores are reported in this section.
He performed particularly well on tasks involving Word Reading. Ds5 achieved a much higher score on this subtest (actual score = 160 ) than expected, based on his overall cognitive ability (predicted score = 127 ). This significant difference is highly unusual and indicates a specific strength in tasks that required him to name alphabet letters and identify and generate letter sounds and rhyming words.
Ds5 achieved better than anticipated in mathematics. His Mathematics Composite score (150) is much higher than anticipated for a child with his general cognitive ability (predicted score = 137). The difference is significant and highly unusual, suggesting that this is an area of considerable strength for ds5.
I don't know what it means, especially since he was so young -- for my ds, I think he just had academically inclined leanings rather than building or sports urges as a preschooler.