As a somewhat simpler example:

>>> f = {False: False}
>>> z = {0: 0}
>>> f | z
{False: 0}
>>> {**f, **z}
{False: 0}
>>> f.update(z); f
{False: 0}

Though these hairier cases aren't explicitly addressed, the conflict behavior is covered in the Rationale and Reference Implementation sections of the PEP. All of the above examples share code (`dict_update_arg`), and that's definitely intentional. I for one think it would be confusing (and probably a bug) if one of the examples above gave a different key-value pair!

I find it makes more sense if you see a set as valueless keys (rather than keyless values).