That isn't a coincidence. The federal reserve district banks distribute the notes to commercial banks in their districts, and, as worded on wikipedia: "A Federal Reserve Bank can retire notes that return from circulation by exchanging them for collateral that the bank posted for an earlier issue. Retired notes in good condition are held in the bank's vault for future issues."
He effectively came up with a way to reverse engineer the district boundaries.
So in other words, the movement of dollar bills is caused mainly by the federal reserve district banks redistributing them, and not by traveling individuals? That sure makes this map useless. I thought that dark line between New York and New England was fishy...
It's not completely useless. For instance, DC, Baltimore, Philadelphia and NYC are grouped together with only a faint line on the border of districts 3 and 5 and no line of the border of districts 2 and 3, with much stronger lines separating DC and Baltimore from the rest of district 5 and NYC from the rest of district 2.
Also, note the strong separation between Virginia and West Virginia, which are part of the same district, but no separation between West Virginia and any part of district 4.
You can definitely see the effect of the Federal Reserve districts on the map, but there are clearly other important factors.
And this is what happens when an expert in one field things he can solve a problem in another field with blind data analysis without any higher-level though, and makes theory-destroying incorrect assumptions along the way.
Some of the Federal Reserve Districts are divided. For example the "12-San Francisco" district is divided in three parts: Norwest, California and Salt Lake City. Perhaps they are Subdistricts of the Federal Reserve?
And there are some exceptions. For example, the north of Wisconsin and the north of Michigan are joined with the south of Wisconsin and Illinois, but they are in different Federal Reserve Districts. Perhaps there a cultural or economical reason for this?
I can do no more than guess, but I think you are right about the San Francisco district. It is a large district and the notes have to be physically carted back and forth to some distribution center, so it looks like they created three locations to do this - one in San Francisco, one in Seattle, and one in Salt Lake City.
I'd also wager that the relatively small discrepancies in the borders are due to the federal reserve being practical and slightly shifting the borders to be cost effective in transportation costs, to deal with one district having an excess of supply and a neighbor having an excess of demand, and so on. When I look at a lot of faint borders near each other on this map, it tells me that this is border movement caused by these kind of federal reserve actions.
The one thing that is blatantly obvious is that the author clearly did not find natural borders of economic activity as he or she claimed.
He effectively came up with a way to reverse engineer the district boundaries.