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It is less typical for achievement to be higher than cognition, but you also have other factors in the case of your DC. Young children (in particular) with attentional difficulties may not have consistent access to their own skills during on-demand testing, so one possibility is simply that the cognitive measures did not fully capture his ability, due, most likely, to intermittently dysregulated attention during the testing. I should also note that there appears to have been a fair amount of divergence among his index scores. Was there also diversity within indices (between subtests in the same index)?
Low working memory does come up often in persons with ADHD, but some individuals are able to pull their attention together long enough to do well on short memory tasks. In addition, there is a subset of those classified as ADHD-primarily inattentive that is described as slow cognitive tempo (Russell Barkley has some more recent work on this), which the field is still discussing. It may be that this has relevance for your child.
