I like to think of it as a large list of granular, contextual rpg stats.

"You get an average 6 for concentration while in a quiet room between 15-20 deg c. Assuming you've not taken more than 10 points of psychic damage in the last 3 hours and are well rested."

They are, in theory, comparing your scores in a set of somewhat defined contexts to see if , overall, it's causing you problems that might be helped in someway by a treatment that has worked for other people who have similar scores in the same set of criteria.

That sentence being so full of caveats is why I think diagnosis is hard for this sort of thing.