Ah, I hate when achievement glitches like that happen!
It is kind of funny, when I first got my 360 I really got into the achievement thing, having a goal to accomplish and having the evidence that you did it exist outside of that game was kind of appealing and made accepting the challenges feel more worthwhile.
There was that girl I was dating for a couple of years, the one you splitscreened with 005. She wasn't into the achievement thing at all before she met me, even though she was a lot more into video games than I was. Somehow my achievement obsession wore off on her and we both became pretty hardcore achievement hunters. It was a lot of fun working together and helping each other get multiplayer achievements and cheering each other on as we took turns in the pursuit of single player ones. Looking back at all the time we spent on getting a few silly achievements seems like a really ridiculous waste of time, like Mad Moxxi's Underdome riot to name one notorious one (we've got every achievement for all of Borderlands), but we had a ton of fun doing it... well, it wasn't always fun... sometimes it was very frustrating, but the sense of accomplishment and high fives at the end were worth it.
Anyway, after all of that for some reasons achievements aren't very important to me anymore. Not having someone else as excited about getting them as me to work with kind of kills the whole thing. Since splitting up this girl and I still play together from time to time, but the last few months we've both been too busy for games and living a million miles apart kind of took away from the achievement earning buzz.
Not sure why I just wrote all of that. 005's story of Mrs. 005 not getting credit for beating the game on legendary brought back a lot of memories of how fun that sort of thing can be as a couple. You're pretty lucky, 05, to be married to a woman who will not only play Halo with you, but will play it with you on legendary, and not only play it with you on legendary, but play it with you twice on legendary because she is determined to have all those achievements too. It is pretty rare.