Not an observation from the streams, but from the pdf manual:
"Only one source of a specific luxury provides amenities, shared with the four neediest cities in your empire. For example, if you have 2 Citrus resources improved around your capital, only the first will be providing amenities to your empire."
If that is true, it changes quite a lot of things... Or does this assume your empire has no more than 4 cities?
Also: "Each Luxury resource can contribute 1 Amenity to a city. The number of cities it provides this bonus to vary based on the era for the resource."
What's this? What is "the era for the resource"?