Windows is not a real-time operating system, and does not give particularly strong guarantees regarding when it decides to schedule threads for execution
IMO, It's not that Windows doesn't make guarantees... It does make quite a lot of them, actually, but it cannot guarantee what the hardware does. Most laptops and tablet PCs have hardware power-saving features that can't be controlled by the OS, to save power - this is also why it appears as though the OS "knows what it's doing better than you" - the hardware does that, not the OS.
This is interesting: http://msdn.microsoft.com/en-us/library/windows/desktop/ms684247%28v=vs.85%29.aspx