Clinton, Obama virtually tied in new poll

From NBC's Mark Murray:

While most national polls show Clinton with a comfortable lead over Obama and the rest of the Democratic field, there's a new USA Today/Gallup poll showing that Clinton and Obama are essentially tied. Obama "bests Clinton by a single percentage point, 30%-29%, if the contest includes former vice president Al Gore. Clinton bests Obama by a single point, 37%-36%, if it doesn't include Gore."

More: "The survey of 310 Democrats and 160 independents who 'lean' Democratic, taken Friday through Sunday, has a margin of error of +/-5 percentage points... Among Democrats alone, Clinton leads Obama by 5 points, 34%-29%. That's a significant narrowing from the USA TODAY Poll taken in mid-May, when she led by 17 points. Among independents, Obama leads by 9 points, 31%-22%."

From what I can tell, they're saying that moderates and independents are starting to surge toward Obama. Any thoughts on why?