Autism spectrum disorder is as the name suggests a spectrum, a lot of people are from a medical standpoint very mildly autistic - research on the biological underpinning of autism is still in the very early stages so we're not sure what the physical difference is where we can say 'if you lack this' or 'if this is malformed' or 'if this bit is like that' so the diagnostic determination is simply 'do you seem to fit somewhere in the spectrum' - maybe we'll find that there is a gradient of autistic severity linked to something like diminished function of a regulatory hormone somewhere and that indeed these people have a few percent diminished function and that correlates to slightly autistic behavior traits. We might likewise find that there is a fairly simple on/off trigger which these people have but which due to the structure of other brain features doesn't result in pronounced behavior.