I'm not all that suprised. Some orgs are settled well in one region, and not another, or are just starting to expand. For example, during my AI research I looked at Theta Phi Alpha and DPhiE--the former was founded up in Michigan, the latter in NYC. Neither has an alumna presence in Houston, which isn't exactly a "small town".
I'm sure the reverse is true as well: predominantly "southern" sororities wouldn't have a strong presence up north either.
Adrienne (PNAM-2003)