It's kind of a complex phrase. Maybe it suggests that something is more unusual or unexpected than what you'd find in typical fictional stories across the entire world.
Well, 'stranger than fiction whole wide world' could imply that there are real-life events or situations out there that are even more astonishing and uncommon than anything imagined in fiction on a global scale. Maybe it's about how reality can surprise us more than our wildest fictional creations.
I think it means that there are things in the world that are so peculiar and extraordinary that they surpass the strangeness we come across in fictional works. It could be a way of expressing how the real world can be full of unexpected and mind-boggling scenarios.