Because he’s with losing independents, silly. The problem with all of the polling this year is the turnout ratio between Democrats and Republicans. Conventional wisdom is that more Democrats will vote than Republicans but their advantage will be nowhere near the 2008 high water mark. So it’s really hard to discern whose winning because their likely voter polls vary widely in what they are showing in terms of party affiliation. But independents are going to make up somewhere close to a third of the electorate and so the polling among independents may be the best indicator of who is really winning.
Of the polls that appear Real Clear Politics average that give out splits Romney is leading among independents in every one of them
IBD/TIPP: Romney is winning independents by 11. Note that this poll has the race tied so they are showing a big advantage for Democrats in terms of turnout which is highly unlikely.
Rasmussen: I subscribe to Rasmussen so I have the splits. They are showing Romney up by 9 among independents and only up by 2 overall. Again this suggests that they are overstating the number of Democrats that will make up the electorate.
Politico/GWU/Battleground: This poll has Romney up by 8 among independents and Obama up by 1 overall. If Romney is winning independents by such a large margin then this poll is obviously showing a huge Democrat turnout likely.
Monmouth/Survey USA: This poll has Romney up 3 among independents and Romney up 1 overall.
Fox News: This poll has Romney up 12 with independents but only up 1 overall.
If Romney wins independents by 7-8 points as current polling suggests then he’ll win the popular vote comfortably.