There's some controversy over the difference between signature moves (moves that are designed around the concept of a particular Pokemon, usually released with that Pokemon, but it is not unique to that Pokemon) and Signature Moves (mostly associated with a legendary or mythical Pokemon, and "belongs" to that Pokemon, usually only learned by that Pokemon (and Smeargle, who cannot use it)). The differences have degraded over time, as a few Signature Moves have been given to different but related Pokemon (ex V Create). Meanwhile, some signature moves have really never been given to any further Pokemon than the one it was concepted for (ex Water Shuriken).
Realistically, Game Freak kind of needs to make a decision about these poorly defined move classifications, and pick 1 strategy.