Interesting. I haven't seen this in the areas I've lived in. I wonder if it's more of a "big fish" type thing, where people that are the "big fish" in an area tend to tip worse (and expect people to be impressed / overly thankful when they tip well). If you're in a small area, people that are wealthy may very well map to the people that consider themselves to be the big fish. For areas with a higher population, being wealthy is much more common, so doesn't really mark one as "special" in the same way.