The definition of 'local calling area' often has less to do with physical distance and more to do with the amount of calling traffic between exchanges. If there is enough traffic between two 'nearby' exchanges then the pair gets designated as local and any prior long distance restrictions (such as paying more) go away. Do this for dozens or hundreds of exchange pairs in your region and you end up with a semi rules based definition of what is local to what.

On top of the traffic driven rules can be a layer of historical factors affecting calls between exchanges.

If you walked into a house halfway between your house and the exchange providing your Google Voice number, it is quite possible that the owner of that house could call your next door neighbor and your Google Voice number, and both would be 'local calls' for him.

http://www.localcallingguide.com