Symmetrical means mirror image. So not only on both legs, but on the same spot on both legs.
I'm sure that someone will correct me if I'm wrong, but my understanding is that symmetrical usually means it's an internal thing (i.e. hormonal). I guess OCD could be hormonal in nature also-maybe some altered brain chemistry or something. Usually things like fleas, bite wounds, other external things wouldn't be symmetrical-they'd be more random.
I think a vet visit is your only option. If you feel like the vet is just brushing you off, either push the issue or get a second opinion.