Although I have never heard of such a finding and would be interested to know where you found that statement, I can imagine that it is an illusory correlation.
Children with above average intelligence have a higher chance of being reared in intelligent families and going to schools with better trained teachers and the like (I'm of course not saying that this is good or right). ADHD may be identified correctly more often in such an environment, whereas the other children (in 'poorer' environments) run more risk of not being treated rightly because their symptoms are not recognized. So there may be a bias in the medical records.
By the way, Mike Rohrbach, I am still waiting for a reply on Marilyn Manson and Evolutionary Psychology. You can mail it to me at email@example.com if you like.