Windows CPU details are taken from the statistics published by Windows PerfMon counters.
These figures can be displayed separately using the Windows Performance Monitor program "perfmon.exe" which comes with windows.
The plugin uses Windows Performance Counters to retrieve the metrics. The current implementation uses the old PerfMon plugin's code to the values. It uses the Registry API instead of the newer PDH API.
The schema for Win-CPU plugin is already deprecated. However, it still uses the Win-CPU plugin codes under the hood when running CPU plugin on Windows.
Available Performance Counters can be seen in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Perflib