Originating in the Mark Twain quote that "Truth is stranger than fiction, but it is because fiction is obliged to stick to possibilities. Truth isn't.", no matter how weird, freaky, or squicky fiction gets, there will be something in Real Life that is even weirder, freakier, or squickier. When such a thing is reported to other people, it will include a Not Making This Up Disclaimer because Reality Is Unrealistic and we really cannot, or do not want to, believe it is real. Not to be confused with Stranger Than Fiction, which is a movie.