Disclaimer: I'm just googling this too, but with Dottie mostly gone maybe more of us need to pitch in and two heads are better than one, perhaps...
Are you looking at something rather like page 10 of
this? If so, I don't think it's comparing the score that would be expected from his WISC with what he achieved on the WIAT at all. I think it's simply comparing his scores on different parts of the WIAT. That is, I think Difference between Mathematics and Math Fluency is his score on Mathematics minus his score on Math Fluency (I don't know what kind of score, exactly). His is very positive, meaning that he scored much better on Mathematics than on Math Fluency. I think the critical value is nothing to do with him at all, but is just saying what number in Difference should be considered a significant difference for this particular comparison. I don't know what Base Rate means, I'm afraid.
So I think you have a discrepancy recorded anywhere he did better on one part of the WIAT than on another. The 21 categories are all the possible pairs of the 7 parts of the test: Oral Language, Basic Reading, Total Reading, ... Math Fluency. (Order is ignored, so you get 21 = 7 x 6 / 2 .) Having lots of discrepancies tells you that he is a kid with (currently) rather uneven achievement as measured by this test - which you knew. Notice that the way it works, you can get a lot of discrepancies from rather simple causes. E.g. consider a child who has a score on 1 of the 7 parts which is much higher than all his other scores, and a score on another 1 part which is much lower than all his other scores. Every comparison that involves either of those parts is going to come back as a discrepancy - that's 11 discrepancies right there. In fact, is that what's happened for your DS? Mathematics being his high outlier score, and Math Fluency being his low outlier, perhaps?
Overall I think don't panic! Looks to me as though this may not actually be carrying surprises.
