"System Idle Process" (if your tick "show processes from all users") will always show a fairly high CPU % usage even though your performance graph might show say 4% CPU usage.
The Task Manager can be a bit deceiving IMO.
Nothing "deceiving" about that at all.
Ordinary computers do not stop the CPU clock when they've got nothing to do. The computer is still running, and the easy way to design operating systems (at least I've found it so) is such that what it's running when it's got no real work to do is simply a bog standard process, but one that does nothing and has the lowest possible priority. This is usually called something like the "idle task" and the code might consist simply of a "JMP ." instruction.
Normally the task manager CPU percentage used is the amount of CPU power that's doing something supposedly useful, and the balance, to 100%, is what you expect to find listed for the idle task. There are other things that can confuse TM so that the numbers don't add up to 100%, but the idle task shouldn't confuse or deceive anybody. The clue is in the word "idle".