I think they use the ACS database. It stands for American Community Survey, which is conducted by the Census Bureau. They survey 3.5 million ppl every year via mail, phone and/or in-person interviews.

Not saying this is the reason, but as somebody who lived in one of of the counties with an outflow, I've got a lot better things to do with my time than answer survey questions. Mail in...either is in the trash or lost on my desk. Phone...if you don't show up as a contact, I don't answer. In-person...get off my porch!