Kippy13,
The NT shell has various "notions" of idle time. You see a gross readout from Task Manager of the "idle process." Windows Management Information service distinguishes and accumulates information on an "idle" state for its own purposes: there are background processes the Shell wants to launch that are essentially uninterupptible once started. For a good feel for this, see my notes in FAQ faq779-4518
That is why there is an inclusion for interrupting tasks that were started from an earlier idle state; and why a network connection even if active can be viewed as "idle" even though a network monitor would show continuing network activity.
As I said, it is very clever, and there are user definable settings to modify the default behavior. Not absolutely brilliant, but awfully clever.
Bill