It is part of a quote from Mark Twain "Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn't."
So, personally I would interpret it as something that characters in general wouldn't believe actually happened unless they experienced it themselves because it would be far fetched even for a story.
The quiz (http://cap-ironman.livejournal.com/920691.html) that went around a few years ago asking if something was canon or fiction is an excellent example of this. Amusingly there were a lot of canon things people thought were fiction and fiction things people thought were canon.
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"Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn't."
So, personally I would interpret it as something that characters in general wouldn't believe actually happened unless they experienced it themselves because it would be far fetched even for a story.
The quiz (http://cap-ironman.livejournal.com/920691.html) that went around a few years ago asking if something was canon or fiction is an excellent example of this. Amusingly there were a lot of canon things people thought were fiction and fiction things people thought were canon.