According to the latest from Rasmussen and Gallup that is not exactly clear and it certainly does not justify the wide disparity between Party's in the sampling of the media commissioned polls.
"Monthly Rasmussen Reports tracking of partisan trends found that in June 2012, 35.4% identified as Republicans, 34.0% as Democrats and 30.5% were unaffiliated. These numbers changed only slightly from the previous month."
Gallup As of 2010, Gallup polling found that 31% of Americans identified as Democrats (tying a 22-year low), 29% as Republicans, and 38% as independents.[3] By 2011 Gallup found that Americans identifying as independents had risen to 40 percent. Gallup's historical data show that the proportion of independents in 2011 was the largest in 60 years. This increase came at the expense of Republican identification, which dropped to 27%, while Democratic identification held steady from 2011. Nevertheless, more American independents leaned to the Republican Party when compared to the Democratic Party. Combining leaners with each party's core identifiers, for 2011 the parties ended up tied at 45 percent.[4]
http://en.wikipedia.org/wiki/Political_party_strength_in_U.S._states#Rasmussen
My theory is people claim to be independents rather than tell somebody ‘it’s none of your business’.