varies from car maker to car maker, and even some models within a mfr's lineup. My guess is that in some cases, bumpers are painted on a different assy line and painted in batches.
On another line is the car body/doors getting painted. Body/Doors could be a different day, different bay, different paint batches..
Im certainly not "giving tesla a pass", but I'd be lying if I said I didnt see same thing on other cars. (like my wifes Lexus RX350 that I sold last year.)
Also, certain colors will display the issue more prominently than other colors to the human eye