No, this is the real map generated by that data. By this methodology, the top territorial area teams are the Cardinals and Braves, with the Red Sox coming in third. Note that the Common Census site itself says their map is “highly inaccurate and should be understood only as a demonstration.”
You can see that for the “United Countries of Baseball” map (an MLB/Nike marketing creation) several teams have had their area inferred over much larger areas based on just a few data points, or none, while others have actually been adjusted in direct contradiction to the data.
For example, northern New York state had many more Red Sox respondents than Yankees, but the UCB version was drawn to show virtually all of New York as Yankees (or Mets) territory.
For my area of southwest Virginia, the actual data showed the Braves as the clear favorite, with the Orioles and Nationals a good ways back, and then small shares of support for the big “nationalized” teams (Yankees, Red Sox, Cubs, Dodgers) and the adjacent regional teams north and west. That makes sense to me, and basically conforms to what I’d have said based on people I know and caps I see around. But the “United Countries” map shows me at the border of the Reds’ and Pirates’ nations–teams somewhere between maybe seventh- and tenth-most popular around here.
In London it’s mostly geographical, influenced by your parents too. If two football teams are leagues apart (literally) it is also possible to support both, though you’d have to choose one if they happened to be against each other in the FA cup or something. For example, you’ll meet a lot of people who support both West Ham and Leyton Orient.
This is a good question. For the American sports, the CC site collected self-selected interwebs drop-ins, and polled affiliations by geography alone. Nobody has done careful polls of large samples that asked the sociological or demographic questions, the factors that may have influenced the geographic distribution.
Same thing here with the Giants & A’s. Both fandoms reach far, but it’s almost all overlapping, with the Giants saturating far more than the A’s.
I expect to be a Giants fan until death, but if I ever move out of state and out of an NL West market, and want to follow a local team, this will be my rule.