They just gave a concrete example: The difference between "we don't know" and "we know that it's zero". Here's another fun one: I've seen APIs where "0.0" was

They just gave a concrete example: The difference between "we don't know" and "we know that it's zero". Here's another fun one: I've seen APIs where "0.0" was treated as "no value, so take the default value". The default value happened to be 0.2.