class Spam:
    bar: typing.Optional[bar] = str

class Spaz:
    bar: typing.Optional[bar] = None

print(Spam.__annotations__)
print(Spaz.__annotations__)

{'bar': typing.Union[str, NoneType]}
{'bar': <class 'NoneType'>}

In Spam bar has str assigned to it and seems like in Spaz bar is assigned None and hence during annotation creation this evaluates to bar: typing.Optional[None] where None is the value of bar and in Spam.bar it's typing.Union[str, NoneType] instead.