We shouldn't really assume anything of those titles. I believe all of them make sense and apply to each of the Organization members' personalities, so it's not "just a name". One example would be Axel, "The Flurry of Dancing Flames", that type of title not only defines him as a fire-based member, but also the whole part about being a flurry describes his fun and energetic personality. Each title means something, and when debating about Lexaeus' title, "The Silent Hero", I believe it defines him as well. Silent perhaps because he was the more closed off and isolated member, when it came down to thinking and pondering to himself, and Hero perhaps because he was very wise and in the Organization had a sort of hero-like personality, very individual, reliable, strong, and just someone whom other Organization members could look up to. Zexion had a close bond with him and perhaps regarded him with high respect, other members could have done the same. That's just my theory though.