You are going at this the wrong way. First off you cant compare a full PC operating system to a mobile device, so lets throw that out of the window.
The question is why is OS X so good at power management? Windows is actually not bad at power management, most people here on Slashdot (even die hard Linux users) will admit that Linux is by far the worst out of the 3 on a PC platform.
As someone else above me stated earlier, Apple have put a lot of R&D into researching battery run time performance. Their inclusion of two inbuilt graphic cards (one high performance and one for general usage) being a big step they took into improving battery runtime. This is why even Windows on a Macbook will not perform as well as OS X because it just havent got support to switch between two graphic cards based on what the system actually needs at that time.
There are obviously a lot more tweaks Apple have done as well, but there are too many to list here...