FS9 Windowed vs Full Screen FPS Problem

Genuine, or maybe not so genuine problem:I discovered today, to my high surprise, when I compared resolutions a bit, that my 8800GT produces in full screen lower FPS, no matter how "big" the picture is in pixels.Meaning, in my windowed mode, I am using effectively about 1550x1100 pixels, when I stretch it to the max on my widescreen, around 1920x1150, and if I use full screen, I usually set either 1600x1200 or 1920x1200.Now, the kicker is, I get more FPS in 1920x1150 windowed maximized than in 1600x1200 (or even 1280x1024!!) in full screen.How is that possible? I thought card is simply rendering pixels, no matter if windowed or not.Is there anything else that is happening "more" when in full screen??Thanks for your help!