Tracking Polls Show Different Pictures of Obama vs. McCain

The Gallup Poll has Obama at +11, and Rasmussen has him at +6.

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Has anyone else been struck by what seems to be a rising discrepancy between the various tracking polls? Gallup currently shows Obama +11, and Rasmussen shows Obama +6. But the Obama margin is notably less in Reuters/Zogby (Obama +2) and Hotline (Obama +1), with Battleground (Obama +4) in the middle. This doesn't cast doubt on the Obama lead. All of the tracking polls but Hotline have Obama winning between 47 percent and 52 percent of the vote. All but Gallup have McCain winning 43 percent to 46 percent of the vote. But the Obama margin in the realclearpolitics.com average has been going down. CBS and Democracy Corps, which if anything usually lean just a little to the Democrats, have Obama +3, not a huge lead. Is it possible there's a little more fluidity in this race than we have come to think?