DNA of fiction: Vish Dhamija

One of my friends recently saw - The Curious Case of Benjamin Button and we got chatting. She had the same argument I had initially - if Benjamin was born as an octogenarian infant, then he shouldn't have become a tot [physically] as he aged backwards.
Would that have made the story any more plausible? Maybe yes. Maybe not.
It's common belief that fact begets fiction. And although the belief is correct, I regard it as only half the truth.
Let me give you an analogy: Take fact as a highly promiscuous mother of fiction. Now wouldn't it be smart-alec to assume that every child she bears would grow up to resemble her? Let's complexify the problem further - would you still predict the same outcome if you're completely ignorant about the father? If the gene pool of the father is, even remotely, as rich and strong as JK Rowling's, then wouldn't it be ridiculous to expect the child [fiction] to take more than a tchotchke from the poor mother? On the other hand, if the gene pool of the father - like John Grisham - is fairly similar to the mother's, you can expect the child to closely resemble the mother [fact] - a tighter, predictable fiction, where everything fits the legal framework.
How implausible or unpredictable can fiction get - depends on the author; how implausible or farfetched should it get, does not have an answer. I guess, that is artistic licence all creative folks have.
People who do not appreciate promiscuity should read biographies and history, though I admit some of the biographies I've read are more whimsical than fiction. As for history - that isn't devoid of fantasies either.