Windows doesn't have the best power consumption--the facts are irrefutable--it's empirically visible.
In the Surface's defense, it is running a wacom digitizer over a 1080p IPS display. Not only is it pushing 2x the pixels as the 11" MBA, but it's doing it over IPS (up to 15% greater power consumption according to Wiki).
But yes, Windows has much poorer battery life, my speculation is that since Apple targets their hardware to a few small (comparatively) number of hardware permutations, they can take this to their advantage, and power-optimize the shit out of everything.
The most effective optimizations will probably be on the driver level, which microsoft would have little say over (in terms of the surface, they should but they're weak in the hardware game).
Remember how everyone used to give Microsoft a free pass on its awful installation process (driver hell) because it supported so many options, but then Knopper (one guy!) created a Linux distro that auto-installed on almost anything. Microsoft makes tons of money on Windows and has tons of engineers, it could power-optimize for a heck of a lot of configurations. Given that a fairly small number of chipsets account for a huge proportion of PCs, I think it would be easier than we think.
In the Surface's defense, it is running a wacom digitizer over a 1080p IPS display. Not only is it pushing 2x the pixels as the 11" MBA, but it's doing it over IPS (up to 15% greater power consumption according to Wiki).
But yes, Windows has much poorer battery life, my speculation is that since Apple targets their hardware to a few small (comparatively) number of hardware permutations, they can take this to their advantage, and power-optimize the shit out of everything.