Macs use power during sleep because it has to power the RAM, but why does a Windows computer still use (more) power when it hibernates? How does it work under boot camp?
It shouldn't use power during hibernation. Hibernation writes the RAM to the disk, and then turns off power completely AFAIK. When the session is resumed, the data is copied back into RAM.
Standby on a windows computer puts the computer into sleep mode, except it does not write the contents of the RAM to the disk. A small amount of electricity is used to power the RAM. This is how things worked in Windows XP at least.