Roof bars C320 and W212

Can anyone help, completely confused by Thule. I have roof bars from my old c320 and I can't see why Thule want me to buy the full kit [bars, fixing kit and feet] to fit my new W212. They imply I need a shorter bar for the W212 which is about 12.5cm wider than the C320. I was hoping that all I would need to do is buy new feet. This I can understand as the W212 [I think] has fixing points hidden under some little plastic covers on the roof instead if the clip round the door frame arrangement on the C320. The MB manual is telling me nothing. The Thule website is equally reticent and has all the clarity of an IKEA instruction leaflet. Why am I surprised. Any wisdom will be greatly appreciated.