It's a multi-dimensional spectrum, it can't be captured adequately by a single number.
I haven't kept up with the current state of understanding, but now I am intrigued and I wonder how I score on those other dimensions they now asses.
Instead of one score, think of it like a cumulative score. You have, let's say, 7 factors all scored from 1 to 5. If the score for being diagnosed with Autism is 21, for example, there are many ways to achieve that. Three areas with a 5 and then a single 3 in any other area would have you diagnosed. Similarly if you score 3 across the board.
Basically, they're saying when you hit that minimum overall score, your factors put you that far divergent from most people that it's at least noticeable.