Problem with Spaces wallpaper

I am on Lion. When I tried to full-screen a pdf document of an academic text, it moved to a new desktop (called Spaces right?) and I think because of the layout of the book, it didnt fill the whole screen and I could still see the background wallpaper. It was a different wallpaper in the background. Why is this so?

I have attached screenshots to make my question clearer. The first picture is Preview in full-screen mode with the old wallpaper as a background. The second is my current desktop screenshot. Thanks in advance