A signature move doesn't mean that only that pokemon can learn it! Would you consider keniesis alakazam's signature move? No other pokemon gets it. Most anime viewers would consider thunderbolt pikachu's signature move, especially now that it has no volt tackle. It's just the move we remember them for, that is and/or goes with their theme, and that is usually just how gamefreak tries to embody the unique abilities mentioned in the pokemon's 'Dex entry. Okay?