One week the people answering (and 1 in 10 here is what we're talking it about) say to themselves 'I think things are going badly in Iraq' and answer one way; the next week, they may think 'Well, we did get Zarqawi,' but they may not have changed their long term thinking at all.  Remember that answers to questions are incredibly sensitive to how they are posed.  Presumably it is just like asking the question with a positive comment attached to the phrasing.