"Who does the poll affects the results. Some. These are called "house effects" because they are systematic effects due to survey "house" or polling organization....

"The largest pro-Obama house effect is that of Harris Interactive, at just over 4 points. The poll most favorable to McCain is Rasmussen's Tracking poll at just less than -3 points. Everyone else falls between these extremes....

"The impact of house effects isn't always so small. Looking back along the trend we see that the red and blue lines diverged by as much as 1 point in late June, an effect due significantly to the large number of Rasmussen and Gallup tracking polls during that time and few polls with positive house effects in that period."

http://www.pollster.com/blogs/how_pollsters_affect_poll_resu.php