My (fully anecdotally) guess based on the google spreadsheet is that it is location of filer based. It appears that folks in the NE go to Vermont, mid-atlantic and eastern south go to Potomac, Midwest goes to Nebraska, deep south to Texas, and west goes to Cali. That's based on the couple of dozen self reported data points on that file. There seem to be a few exceptions to that general rule, but to me it appears to be based on your filing location/local office.