I've heard that these engines spray more fuel into the cylinders during the first few hundred miles to help with the breakin. Not sure if there's any truth to that, but the MPG changes I saw might support that theory.
There's a very small likelihood that that is the case. They want the first impressions of their vehicles to be as good as possible. I'd imagine the MPG changes you saw after ~800 miles were probably because your new car syndrome has abated, and your right foot lost some weight.