Well, it may be that you are used to the accents so you hear the subtleties (can't speel worth a darn tonight). As a Northern, I'd likely just hear the overall accent.

It's like My Fair Lady. Henry Higgens could hear the difference. I just hear an English accent. I can recognize cockney because it's so very distinct just like I can recognize upper Bostonian from New Jersey accents (having lived in both areas) but telling accents apart when you aren't around them a lot is hard. So I tend to lump them together into regions.

Actually, I'm curious. If you watch the Clone War cartoons on cartoon network, what is the mascot's accent (he shows up to introduce the next part - looks like a metallic doughboy)? Sounds southern to me.