When it comes to "geographical locations" I think they mean the bulk of the testers were chosen because they lived:

a.) in Japan
b.) in the western U.S.
c.) in the U.K.

This was done because that's where SE's three main offices are located, and they likely wanted to have as many people online slamming the servers during normal business hours as they possibly could.

Yes, I know it would make more sense to have people on the U.S. east coast and Germany in the alpha, since the afternoon/evenings in those places happen during normal business hours in El Segundo, CA and London. But that's not how managers think.