I think for that they were trying to hype up the game when the story was falling short so they could still get sales. and they probably changed the campagins story a bit to work with certain mechanics in the game, like having ai squad members with you. You could tell from and E3 footage before the game came out, it was unfinished at least story wise. My main guess is that it was a few execs or some advertisement team wanting to provide something that gets people wondering what "big stuff" they had planned. People liked the idea of the mystery, but the writers couldn't figure out anything other then "oh its for cortana lmao"
Master chief was by himself at first, then a different one where he has a squad, one of the squad members was changed out, etc.
Also to perhaps try to make their new character Locke seem like a badass bounty hunter, cause clearly they were trying to kick master chief out of halo. Seeing how he only had 2 missions where you could play as him. I