I made a thread about my personal theory with this:
http://www.gamespot.com/mass-effect-3/forum/there-is-a-canon-ending-ending-spoilers-62559127/
Basically, it think the last scene locks in one of the endings as cannon (based soley off of the framing of the end), but whether this was intentional or an oversite is yet to be seen.
As for why the're still human after the green blast ending (or why the scene is identical after every ending for that matter), I think that it's supposed to be covered by his "a lot of the details were lost through the years" comment, it just failed miserably to do so.