They don't, 37% of the people without collage degrees are never unionised for example. They do in fact track how large fraction of the career is spent in a unio

They don't, 37% of the people without collage degrees are never unionised for example. They do in fact track how large fraction of the career is spent in a union job, there's none of these exclusions you've imagined for some reason. The emphasis on "persistent" comes from the fact that most of the total gains arise in the later parts of a career, so if you spend less than 49% of the career in a union there's not as much benefit at age 65 (if you lack a collage degree, if you have a degree it is on average better to not join a union).