It’s crucial to note that our bias scale is calibrated to the political >spectrum of the United States, which may not align with the >political landscapes of other nations.
From their methodology page.
They aren't rating for any and all bias. They are rating for political bias within the context of the US political landscape.
The President appoints people to the USPS Board of Governors for a term of 7 years. That board selects the Postmaster General.