ADHD, as a diagnosis, is a bit like IBS in that when there is something going on but the clinician does not know what it is, they go with these labels in many cases.
This does not mean that it is being 'wrongly diagnosed' because the symptoms can be quite varied within the spectrum, so it would be more accurate to say that ADHD is poorly, or rather widely defined.
There is, however, an incentive to diagnose this condition since it triggers extra funding for the fundholder GP. Extra funds also become available to the school and the child's family.
What surprises me more than anything about ADHD is the official NHS front line advice about it: Unknown cause and management using drugs. There is a large amount of evidence pointing to sugar, excitotoxins and other dietary causes for the symptoms associated with 'ADHD', and similarly successes for treating these symptoms by modifying the diet.