The additional achievements look to be the same ones that were leaked from PAX a few weeks ago which are the ones Bungie made it clear they wouldn't be talking about just yet. It's interesting to note that a few of these newer achievements make mention of "Mythic" maps, referring to six different (and yet to be announced) multiplayer maps called Assembly, Citadel, Heretic, Longshore, Orbital and Sandbox. Or so we think they're multiplayer maps.
Whatever is going on, we're totally out of the loop because Bungie technically only announced that the ten Legendary achievements would be included in today's H3 update. Maybe the posting of additional 500 Gamerscore worth of achievements was a mix-up by the Xbox.com folks OR maybe the Superintendent has something to do with this. We're confused, but REALLY excited!
[Thanks, to everyone who sent this in]