I didn't see it that way previously with regards to curators deciding to apply this mark or not.
Basically, some curators who curate rather randomly, apply certain filters. The content length is one of them, have one (sometimes multiple) images is another, the reputation of the author or how long they've been on the platform could be another. Using or not using certain tags or publishing/not publishing in certain communities, or through certain front ends are another set of filters they might use...