This. Carriers would distort any phone-based metrics in the US, as there are certain areas of the country where AT&T isn't available, and thus had minimal iPhone market penetration until the 4 came out on Verizon.
AT&T didn’t have much for rural coverage in northern Minnesota and many parts of North Dakota, but got much better with the purchase of Cellular One (no surprise; add needed towers, get reception, get users).
However, that acquisition was a while ago. I’m sure nothing is overnight. Still, not much is truly revealed about these numbers. This article still leaves me unsatisfied.
Good observation, correction for coverage maps is a good idea. Another correction which might be relevant on a smaller-than-state scale would be distance from nearest apple iStore.