With strangers, I've found that it is often not worth the while to give an unexpected, albeit truthful, answer.
I recently came back from a whisky tasting (or nosing as some have corrected me). I stopped to fill up at a gas station on the way home. The attendant looked at my kilt and said, "You must be visiting from Scotland!" Rather than launch into a story about my ancestry, the fact that I only lived a couple miles away, and why I happened to be kilted, it was just so much easier to say, "Aye," and pay for my gas.

If they really want to know, they'll ask further. Otherwise, they're only asking to impress their friends, and who cares what a random stranger thinks?

Still I think you handled it well. Remember that the uneducated speak without thinking. It enables the rest of us to quickly recognize them as idiots.