[irb]

'SF' is very commonly used as an abbreviation for science fiction, particularly within the community, and has been for many decades. It's not a further abbreviation of sci-fi, it's an alternative.

One of the intentions is to distinguish it from futuristic fiction which is more about the rip-roaring adventure than the science. For example, Star Wars would be labelled sci-fi, 2001 would be labelled SF.

If you're thinking it sounds like there's an element of snobbishness in there, you are definitely right. It's much the same as with those authors who prefer to say that they write speculative fiction rather than science fiction (SF as an abbreviation also has the advantage that it works for both of those).