Mental disorder diagnostics are all 'Design by committee'. People get together and vote and they are argue and they create categorizations based on that. They do their compromises every time they need to update the book.
Diagnosis is then based on observed behavior and interviews. They don't do some sort of blood test, MRI scans, or X-rays or any other kind of medical diagnosis. Yeah, sure, there is some sort of questionable neuroimaging scan they can do, but none of that stuff ever gets used to actually diagnose people.
In terms of a science it's much closer to the 'sociology' side of things then 'search for the cure for cancer'.
The original purpose of DSM was so that psychologists and the lot have categorizations handy for filling out claims with patient's insurance. It's a lot easier to convince insurance companies to pay for treatments when the diagnosis is 'Attention deficit disorder' rather then 'Little Timmy is having a hard time sitting still in class and occasionally gets completely out of control'.
That doesn't mean these disorders don't exist or that DSM diagnoses are invalid or useless. It just means that it's all a bit 'hand wavy'.