Dark,
You make a good point. However, the greatest JW source in the U.S. is the immigrant population. South Dakota is definitely not a state with a big immigrant population. So the average that you are using is quite likely not representative of South Dakota. The "white" population of South Dakota is far less likely to be a JW than, let's say, the heavily immigrant population of los Angeles.
The Latino population, for example, brings far more publishers to the watchtower than just about any other group. The Latino population in SD is only 3%. I guess SD didn't enjoy the big surge of Latino publishers that I witnessed in recent decades. Therefore it is fair to infer that ratio of JW to population in SD should be lower than the U.S. average