In my opinion, neurology may not necessarily have the best vantage point with respect to ADHD. Not that I'm in any way knocking neurologists, mind you, it's just that the theory, research, classification, assessment, and diagnosis of ADHD has generally been spearheaded by psychologists owing largely to proficiency with appropriate psychological testing techniques.
There definitely have been structural neuroanatomical differences found between children with and without ADHD; unfortunately, most studies do a fairly horrible job of differentiating between the various ADHD subtypes (most include a mix of combined-type and hyperactive-type) or even effectively establishing the presence of ADHD, tend to have more significantly more males than females, and often ignore the potential role of comorbidity.
I'm not saying that ADHD isn't caused by ventricular asymmetry (although I have my doubts, and believe that more substantial evidence exists for involvement of dopaminergic and glutamatergic subcortical-cortical neuronal pathways). However, I think that with the way the research currently stands, we have absolutely no idea what causes it, although we do have a few educated guesses. I will say that based on the location of the lateral ventricles, and their proximity to the subcortical origins of the aforementioned neuronal pathways (particularly the neostriatum and cingulate gyrus), it makes sense that asymmetry could show up in children with ADHD.
It'd be similar to saying that schizophrenia, which has frequently been associated with prefrontal atrophy, is in fact caused by prefrontal atrophy, when in fact we don't know if x caused y, y caused x, z caused both x and y, or x and y are only coincidentally related.