When you factor in that a vast majority of independents vote against a sitting incumbent, you can add most of the "undecided" to Romney's numbers. Take the 51-35 poll...that leaves out 14%. If, say, 10% vote for Romney and 1% for 3rd party candidates, then it could be 61-38 in favor of Romney. An enthusiasm gap could widen this.
As for Rasmussen's poll of all likely voters showing 49-47 in favor of Romney, that's 4% leftover. 2% to Romney, 1% to Zero, 1% to 3rd party. 51-48 in favor of Romney.