After hours of searching the Internet, finally I had to settle for the MSDN documentation. According to MSDN for XP we have :
Global User Power Policy : Contains global user power policy settings that apply to all power schemes for a user.
Global Machine Power Policy : Contains global computer power policy settings that apply to all power schemes for all users.
User Power Policy : Contains power policy settings that are unique to each power scheme for a user.
Machine Power Policy : Contains computer power policy settings that are unique to each power scheme on the computer.
And for Vista+, there are no global/local or user/machine settings; instead, everything is scheme specific and the settings apply to all users. (See this)
But when I tested aforementioned facts on two different machines (one running on Win XP and the other running on Win 7), I got unexpected results! On XP, running my code from a windows service (running as SYSTEM account) and a windows app (running as an administrator account) indicated no user-specific behavior. On Win7, the same experiment, created the same results!