This is incorrect in many ways.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1525089/

Attention-deficit/hyperactivity disorder (ADHD) is a behavioral disorder that affects up to 1 in 20 children in the USA. The predominance of American research into this disorder over the past 40 years has led to the impression that ADHD is largely an American disorder and is much less prevalent elsewhere. This impression was reinforced by the perception that ADHD may stem from social and cultural factors that are most common in American society. However, another school of thought suggested that ADHD is a behavioral disorder common to children of many different races and societies worldwide, but that is not recognized by the medical community, perhaps due to confusion regarding its diagnosis and/or misconceptions regarding its adverse impact on children, their families, and society as a whole. In this article we present the available data, with a view to determining the worldwide prevalence of ADHD. A total of 50 studies were identified from a MEDLINE search for the terms ADHD, ADD, HKD, or attention-deficit/hyperactivity disorder and prevalence combined, for the years 1982 to 2001. 20 were studies in US populations and 30 were in non-US populations. Analysis of these studies suggests that the prevalence of ADHD is at least as high in many non-US children as in US children, with the highest prevalence rates being seen when using DSM-IV diagnoses. Recognition that ADHD is not purely an American disorder and that the prevalence of this behavioral disorder in many countries is in the same range as that in the USA will have important implications for the psychiatric care of children.

To address your point about the diagnosis' dependence on severity, here is a repost of an explanation I wrote in a previous comment, as I don't think I can say it any better than this:

You can try to simplify things to overdiagnosis, to loosened criteria, to disorders not really being "actual" disorders, but nobody really seems to understand that disorders are defined by suffering, impairment, and dysfunction, not just a trendy label. Many people say, oh, the criteria for condition X is so loose, everyone experiences that, but the disorder isn't defined by "I have this issue once in a while", it's defined by "I have this issue near-constantly, it's massively impairing my life, I'm suffering, it's causing immense problems, it's a consistent long-term issue that does not abate". It's defined relative to your age group, not in absolute terms. A toddler having trouble focusing and running around is normal. A toddler with immense difficulty focusing and constantly bouncing off the walls is not. A child feeling sad is normal. A child who is constantly sad, wants to kill themselves, and lies in bed all day is not. A child with an overly-active imagination is normal. A child who is constantly hallucinating, delusional, manic, psychotic, etc is not.

Reliance on any single non-specific test is not a good solution, especially something like Raven's progressive matrices. The best way to diagnose ADHD is by combining numerous evaluations, including a range of psychological tests that could highlight potential symptoms (such as the TOVA). Comparing pre/post medication results is not a good way to diagnosis anyone for numerous reasons (medications will often improve performance in everyone to some extent in the short term, many people respond poorly or not at all to different medications/doses/release mechanisms, etc -- in short, response to medication is not indicative of ADHD, and it's incredibly unreliable).