They compared with different "landmarks", and those are usually built around where people are, or people settle around where the landmarks are. I also found it strange that they didn't at least share a hypothesis of their own. But that would be mine -- settlement density and "modern lifestyles" may correllate with some negative health effects, or on the other hand with growing older and therefore being more likely to die of cancer? Maybe they'd have found a negative correllation if they tested for distances to deserts, open plains, and mountaintops.