As mark said above this image is 11017px wide but it has black lines part way down each side making the gray center part 2px smaller than the top section which has no black line. Remove the black lines from your image and the problem should go away. You can easily check this by simply removing the reference to the image in Firebug.
However you would seldom want a background image created like that because it dos not allow for resize and fluid content. Why not just use a background colour and css borders as you don't have a gradient effect going on in that section anyway?