This is a 100% legit phenomenon. Volunteer plants are typically more robust because out of all the seeds that are scattered, few sprout plants that are robust enough to survive in adverse environments without care. I once saw a corn plant randomly growing in a parking lot outside a movie theater, and my thought was that someone tossed their popcorn onto the ground and one of the unpopped kernels hadn't been killed by the heat. As a gardener, when I spot a volunteer plant I like to carefully dig it up if I can and plant it in my garden and give it ideal care. These are always my biggest and best producers.