The problem is that often a child with ADHD may also have autism, or various other difficulties (learning disabilities, anxiety disoder) that may mask, or have similar symptoms. And then you can also have such disorders without having ADHD. The same structural differences in brain can cause lots of things.

Especially clear if you have one who does and one who doesn't.

Absolutely. Take my two kids, they are wildly different,and so equal. One can focus a bit the other is impossible to get focused. He gets distracted when doing a wee, is astonishing, sometimes I'm speechless.

Even if true, the problem is those who don't but think they do.

I suspect you might also have parents who kinda like the idea that there child's behaviour can be explained by ADHD. My sister's kid often gets in trouble at school for example, and whenever this happens she will try to excuse it by saying she has ADHD. I'm not at all convinced given that imo it's far easier to explain the misbehaviour with bad parenting.