You are generalizing from your own area again.
Three Walmarts here; identical pricing. Some differences in floor space and inventory. There is another Walmart about 40 miles away in another state where the prices are very slightly different, but they have more local produce.
Also, large cities in this part of the country are nothing like large cities in your part, in terms of grocery pricing, demographics, and just about anything else.
Look at Walmarts own corporate statement which even admits to price differences between stores.
http://corporate.walmart.com/frequently-asked-questions
Why is there a price difference between Walmart stores?
Prices may vary from store to store because each store is required to manage its own inventory. Therefore, a manager may lower a price to clear out an overstock item, to reward customers with an in-store sale, or to compete with local merchants. It is not our policy to price match our own stores or our online service since we are not in competition with ourselves.
If you would like more information on pricing policy, you may contact your local store management.