Okay, so after just watching the clip, I have to say Jimmy Fallon sucks at Halo. However, he is the lucky bastard who got to play the campaign first, outside of 343 (according to Josh Holmes).
http://www.latenightwithjimmyfallon.com/blogs/2012/06/jimmy-and-josh-holmes-play-halo-4/#.T9gfqGiG1qM.twitter