Because, like it or not, Your clan and clan chief do not own the name MacGregor, nor Gregor, nor does your Clan Chief have the weight of law behind him when it comes to tartans, thus anyone can make up any tartan they like and call it (just about) anything they like, including MacGregor dress, hunting, weathered, etc. There is, at the moment, nothing your clan chief can do about it, save for telling the world which tartans he deems to be official and accepted.