I hope he won't take offense at my dissecting it, but he sounds more ... hrm, I'm not sure what the accent is... it reminds me a bit of Dave Sifry, actually. You could really hear it when he said "so" in this video, which tended to be a bit drawn out: "seeeewwww". I expected something a little bit more clipped and neutral.

Perhaps the difference is that he spends a lot of time getting his essays 'just right' whereas speaking is a bit more of an off-the-cuff affair.