The diag criteria are an ad hoc thing that only exists so that the society has a systematic way of deciding wether a given individual is autistic or not. Someone who just barely misses the criteria to be positevely diagnosed could very well have a lot in common with those who meet slightly more criteria.

Think of it like the administrative criteria to be considered "poor" in a given country: it helps to decide who can benefit from financial help and such, or to have statistics on how fair is the ressource distribution through the time, but it doesn't mean that your life will switch the very moment your income crosses the limit.