Probably because of this angle which clearly shows Chamakh shoulder barging Arteta over and then falling over Arteta's feet as he falls.
http://farm4.staticflickr.com/3767/1...d64cf232_o.gif
And look where the ball goes, way out wide. He'd have had a lot to do to get there and centre the ball, there's no way to be certain that defenders wouldn't have been covering.