They heard we like taglines so they put taglines in their taglines so they can market while they market.

Seriously though, it probably hints that they may do other "Anniversary" games in the future. That's the only logical conclusion I can come up with. Well, this is MS/343i we're talking about. Nothing they've done thus far has been logical, and if anything, it's all been destructive to the franchise and the people who have really worked on it.