And not always in this situation. Dr. Pepper is distributed by the same local company that distributes Pepsi but I've never seen the Heritage (not that I've looked, don't like Dr. Pepper, but that's a pretty noticeable label).It wont be available in regions where Dr. Pepper is distributed by Coke. Pepsi has an exclusive contract.
I don't know this for certain, but from a branding/advertising perspective these things are usually centrally directed. A company generally wants to present a consistent image across markets although there can be a lead-in time. For example it took a long time for the new Coke bottles and caps to make it down here. I bought one in Denver about a year before I ever saw one locally. I imagine labeling can also be subject to test marketing.Do you know if the bottlers responsible for making/producing their own packaging/labels?