Bit of a clue here Colin
http://www1.skysports.com/f1/news/12472/9243875/revealed-how-mercedes-packaging-of-their-turbo-engine-has-given-them-the-edge
Mercedes' customer teams all have the same advantage. However, because McLaren, Williams and Force India only took delivery of their power units relatively recently, they have had less time to work the layout into their respective car designs.