The CPU plugin collects the amount of time spent by the CPU in various states, most notably executing user code, executing system code, waiting for IO-operations and being idle.
This since has been an FAQ: The CPU plugin does not collect percentages. It collects "jiffies", the units of scheduling. On many Linux systems there are circa 100 jiffies in one second, but this does not mean you will end up with a percentage. Depending on system load, hardware, whether or not the system is virtualized and possibly half a dozen other factors there may be more or less than 100 jiffies in one second. There is absolutely no guarantee that all states add up to 100, an absolute must for percentages.
- Mac OS X