The expression “truth is stranger than fiction”, or at least the overuse of it, is a prime example of lazy thinking. The truth is, truth is only stranger than fiction by virtue of its being true. Reality has an unfair advantage over fiction in this regard. If an outlandish piece of fiction, whose strangeness does not otherwise particularly strike any reader, were to occur in real life, people would still say that truth is stranger than fiction. But if it followed the fiction to the letter, how can it be stranger?
When an extremely strange series of events takes place in reality, a quite natural reaction is to comment that “you couldn’t make it up”. If it is truly strange, it is unlikely that someone would have made it up (simply statistically speaking), but it is certain that someone could have made it up. After all, most strange occurrences are the results of human thought and action, and fiction is no less (in fact even more) the product of these things.
Perhaps, then, we should ask: “what do we mean by strangeness?” It seems certain that this is different for reality than for fiction. Perhaps strangeness is simply the feeling that arises when we perceive that something has gone beyond its “natural bounds”. Reality is bounded by many laws—of physics, of psychology—and so if something appears to go beyond these bounds it will give the feeling of strangeness. But the bounds of fiction are much harder to gauge. Ostensibly they are at the whim of the author, and so can be absolutely anything. There are of course the rules that come from narrative convention, but most readers will be aware of a wide enough variety that nothing in this regard will be surprising, at least not surprising enough to engender the feeling of strangeness.
I would submit that the overriding boundary of fiction is that, for the most part, we just don’t believe it. If fiction can make us believe (however different the nature of this belief is to that of real things), then it creates the feeling of strangeness. Reality is the opposite: if it makes us doubt, if it makes us think that it’s a fiction, then it has become strange.