Well, they're not trying for a fine differentiation. American English, broadly speaking, does fall into four major dialect areas, Northern, Southern, Midland, and Western, but each of these has plenty of subdivisions. (One list I saw distinguished thirteen different "Southern" accents, for example.)

Place of birth or residence doesn't have a lot to do with it, either. I've never lived in the South, and didn't live in the Midlands until taking up my present job - but I'd already noticed that my dialect was of that type years before. (My mother's family has some Southern roots, but the last time anyone in my family lived in the Midlands was three generations back.)