I think that this idea of campus overlap also has a lot to do with where your sorority is strongest geographically.
Compare D Phi E and Phi Sigma Sigma
D Phi E has 18 NY chapters, 13 NJ chapters and 13 PA chapters.
Phi Sigma Sigma has 21 NY chapters, 9 NJ chapters and 16 PA chapters.
Obviously we all are stronger in this area, and of course, will have overlap.
Now, look at Theta, Kappa and Pi Phi:
Theta has 16 CA chapters and 7 TX chapters
Kappa has 13 CA chapters and 7 TX chapters
Pi Phi has 13 CA chapters and 7 TX chapters
Ahhh, correlation
