I'm going to say both. It would explain why I can never get excellent as option. Seems to be just bad wording on Googles part and lack of the explanation. They are probably using Good to specify 1080 is possible and excellent to specify 4k is possible. But they seem to be checking the resolutions you support so only go up to 1080 if that's what your tv or monitor support. So to save bandwidth and the like they only send 1080.
Would be nice if they gave us resolution and connection quality separately though (with a disclaimer that 4k is only possible with excellent or something perhaps).