Considering dev kits are basically pcs and I'm sure there have been several different iterations of the dev kits themselves I do not see any reason why they couldn't. It's entirely possible they had dev kits with ddr3 and a different dev kit with gddr5.
Now my personal opinion still holds that MS does not care about competing on a technical level anymore. They hit a home run with Kinect and are banking on that transitioning into this gen. It makes me sad to think about it.
Home run in what sense, wasnt software sales shocking on kinect?. The core gamer hates it.