Mark Twain learned how to tell a story by listening to verbal masters of the art, around campfires, in wooden huts, and in stores and bars. Then he transformed this knowledge into print. Twain was not, strictly, speaking, a novelist, philosopher, seer, or travel writer, though he was a bit of all of these. Essentially he was a teller of stories-- a teller of genius-- because he was ruthless.
Twain grasped, even as a child, the essential immortality of storytelling. A man telling a tale is not under oath. He must insist, indeed he must insist, that his story is true. But this does not mean that it is true, or that it needs to be. The storyteller's audience may expect him to proclaim his veracity because that is one of the conventions of the art. But what the readers or listeners actually want from him is not verisimilitude or authenticity but entertainment and laughter. They know it, he knows it.
When he says, "What I am going to tell you is strictly true," he is merely pronouncing a formula of the genre like, "Once upon a time." A storyteller is a licensed liar, though he must never say so.
When confronted with Thomas Carlyle's assertion, "The truth will always out at last," Twain replied: "That's because he did not know how to lie properly."
Paul Johnson, from his essay "Mark Twain: How To Tell A Joke" (Published in The Creators, Harper Collins, 2006).