This is explained in the “How should I interpret the Userpop reports on the web?” section where it says “The
rows of the report generally specify where the registrant ‘resides’. Where a person resides is based not on their mailing address but on their ‘community of residence.’ Because we are a highly mobile society, it is to be expected that each Area will report registrants ‘residing’ all over the country.”
The Userpop of a given area will report
all of the people who have ever registered in that region by their residence. Some of an area’s registrations will be visitors or people who for some other reason report that their community of residence is in another region.