There are huge regional variations to rates of ADHD diagnosis. For example, it's 3.5x higher in North Carolina vs. Nevada. There doesn't seem to be any reason why this would be given that ADHD is believed to be almost entirely genetic.
It's frustrating to think about the lack of consistency in ADHD diagnosises and treatment. In /r/ADHD (ADHD subreddit), some people get diagnosed after a few minutes of talking with a psych. Others live in countries where ADHD isn't even considered a thing. Many went through the same rounds of tests that I did. Even after diagnosis, I had the same problem as several /r/ADHD members in that the first psychiatrist I went to looked at my psychologist's diagnosis and was like "Meh, you got decent grades in high school anyway. You're just stressed/tired/depressed/etc." Wasted my time with four months on four different depression medications before I got a second opinion.
Overdiagnosis is almost a cliché. This might happen in some places. There are just as many that the diagnosis is painful and thorough.
Also, you're assuming two things: 1) that genes are evenly distributed among the population, which they're not. And, 2) that diagnosis and treatment options are the same everywhere, which they're not.