Because the central character in the 1946 A. E. van Vogt novel Slan was a superhumanly intelligent youth in unsympathetic surroundings, "Fans are Slans!" the unserious claim to slanhood, became a rallying cry among Third Fandom.
Fans identified with slans as a persecuted minority because of the reactions they frequently got from mundane society merely for reading that "Crazy Buck Rogers Stuff" or even reading at all – but not to the extent that they really believed fans were superhuman beings, Second Fandom's half-serious Star-Begotten claims, notwithstanding. Still, during the 1940s and '50s, when many fans were intellectual autodidacts working in blue-collar jobs, there must have been a certain comfort in thinking themselves superior to the mundanes around them.
|from Fancyclopedia 2|
|Literally understood, "fans are superman mutants" — or, at least, "fans are smarter than most people" — but actual semantic content is according to the mood and attitude of the user.