Well, for one thing, it's something publishers can put on books so that the reader (especially of beginner books) has some "assurance" that the author is qualified to write on the subject. Then again, people also write under pen names that include honorifics like Ph.D, with or without the requisite education. While some people for whom the title Tarot Grandmaster is used certainly would qualify as such, I also think that it'd be rather easy for a less scrupulous character to claim this title deceptively, especially in this very subjective field of study. Since there's no one universally recognized certification body or system in Tarot, I would be skeptical about the use of this title beyond a select few individuals - and you know already who they are without seeing Tarot Grandmaster after their names.