When browse the Health Monitoring, Apache CPU usage shows more than total CPU usage percentage in Health Monitor. In running TOP, the total CPU usage seems more accurate. Why is so?
The mode of calculation which uses not absolute percentage.
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.
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.
That is why some values can be different. For more information please visit http://collectd.org/.