From Board 8 Wiki

Same Fanbase Factor is the generally-accepted theory that in matches involving two characters that share the same fanbase, the results will be somewhat skewed in favor of the more popular of the choices. Most matches involving two Square, Nintendo or Sega characters are said to have some degree of SFF.

SFF is also sometimes called the Something Fishy Factor, to make light of the fact that whenever anything the least big odd happens in a poll, Gurus will attribute the oddity to some kind of SFF without hesitation (eg "this must be RPG SFF" or "clearly he's getting PSX-SFFed here").

Variant Factors based on this same idea include the highly debatable rSFF (reverse SFF, where the 'weaker' character overperforms) and the frankly unmistakeable LFF (leach fanbase factor; only applicable in polls with 3 or more choices).