Dear Neville,
I didn’t look at the study yet, but two easy points.
1. Those cities in the Midwest are all on the small side. (Maybe except for Chicago.) One can know every shul and they only have a handful of kosher establishments.
2. It is more likely that one that intermarries, marries later and has much less kids. Those who marry-in in an assimilating society see the need to repopulate.