It’s essentially a measure of income and race. San Francisco has high incomes, and high Asian population. Obviously richer people live longer, and Asians have lower rates of obesity and longer lives in average.
Brownsville is much poorer, and pretty much entirely Hispanic. Obesity and poverty rates probably much higher.
It seems like it was intentional by the authors not to control for it, which I think is fair depending on what you are trying to show, but it’s irresponsible not to mention it.