That's a misconception. Prices are not the same across stores.
I live in Los Angeles. Many times I've shopped at the TJ's in Silver Lake and one of the TJ's in Pasadena on the same day. Most prices are the same, but on many items the Silver Lake store is consistently 5-10% higher.
I've also shopped in midwestern TJ's and noted that the prices were generally lower than LA.
Products differ significantly, too. Items with the exact same name and packaging can be totally different regionally. For example, "Sonoma Chicken Salad" used to be a favorite of mine here in California. The Iowa version was disgusting, with roughly twice the mayonnaise, fewer nuts and grapes, and 3x the sugar.