It's confusing, but "idle" is a priority class as well as a state: https://docs.microsoft.com/en-us/windows/desktop/procthread/...

The idle task (when there's nothing to run) doesn't have a priority at all, it just gets run when nothing else is available. the "idle" priority is an actual priority, for when there's no higher-priority task to run. The ZPT is prioritized below "idle", so super-duper-low priority.