Funny that no one has been able to actually see the transcript of those "brilliant grades".
Well, I’d guess that with the prospect of being named Ambassador to Belize, Thummalapally probably tried to give the impression that Obama was a regular guy, likeable, and brilliant. Obama had already mentioned his drug use, but this put it in the best perspective and likely would appeal to many youthful voters or nostalgic hippies.
Perhaps he wasn’t cautioned not to mention that he was Obama’s roommate, or perhaps he hadn’t realized what that might give away.