Like several people have said, I think it’s more down to the car/tyres than anything else. I’m guessing Hamilton is having to drive at less than 100% to preserve the tyres, even over a single lap, something he’s obviously not very good at considering his ‘I cant go any slower’ comments on the radio. It could also be that Hamilton is concentrating more on fixing his tyre problems too, trying different lines etc while Rosberg goes for outright speed.
Having said that, it is a surprise that Rosberg is beating Hamilton, considering Hamilton’s record against Button in the last couple years. I always considered Rosberg to be a Button-like driver, I guess on this evidence his one-lap speed is greater though, more like a Raikkonen.