Then I don't know what to say...
I just tested it to be absolutely sure, and there's no difference in sensitivity or field of view if your resolution is 16:9 and you switch between auto and 16:9. If you have a 21:9 monitor and switch to 16:9 you will get a 16:9 image with black bars, and the sensitivity will be exactly the same. You just see a bit less on the sides.