and only lose 1% of battery charge in 48 hours--with a first-generation model. Why?

Yes, I would have to say it's in the software, but it's also in the design of the device. An ARM or Atom processor doesn't draw as much power as an x86 and as such automatically extends the life of a given battery by some factor. It has already been reported that the Surface RT gets roughly 8 hours of battery in use while the Surface Pro gets about 3 hours. Assuming all else the same for just the moment and we see that the different processor alone makes a huge difference in performance. By turning things around and jailbreaking the SurfaceRT, you find you get access to all of Windows 8--but desktop performance is abysmal.

In other words, it's not just software that gives longer battery life, it's the combination of hardware and software--the engineering of the device along with the operating system that controls how well a given product will perform. This is also why the so-touted "tech specs" are such a poor measure of how well a machine will work for any given individual.