Somewhere down the back of my couch are a whole bunch of coins. No-one will ever know how many there are, because nobody is brave enough to stick their arm down there. The uncertainty here is epistemological: it is a fact about our knowledge of the world, not a fact about the world itself. There is some specific, unambiguous number of coins down there, we just don't know what it is.
By contrast, imagine looking up at a sky full of clouds. How many clouds are there in the sky? It's impossible to say, even though we can see the sky very clearly, because there's no clear way to decide where one cloud ends and another cloud starts. The uncertainty here is ontological: it is a fact about the world itself.
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