I'm not sure I agree with what the author is trying to imply. The three-fold increase in metro areas, sure, but the wider spread of geographical preference? I'm not buying it. For the 1970 data, the 5pt spread represents almost a 42% change from the average. In 2010, the same 5pt spread represents only a 15% change from the average. If we were to apply a 42% threshold for orange/black squares on the 2010 data, it would be +/- 13 points, making the distribution reasonably similar to 1970.