>>487263I think the subtitle helps people distinguish at this point with X, V, and other roman numerals in titles.
I tried to convince someone in irc that neptunia VII was the 4th neptunia game and not just neptunia 7 in roman numerals and they legitimately did not believe me.
So my best guess is it's there to help people figure out that it's not xenoblade 10, but a spinoff of xenoblade which itself is xeno-5.
I'm sure there's someone in marketing at treehouse that has really good reasons for this better than what I can provide, but I think that's part of it.