[ubuntu] 4 CPUs?

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http://ubuntuforums.org – I have a dual-core Core i3 on my laptop, and System Monitor gives usage statistics for 4 different "CPUs". I'm assuming that the 4 CPUs the OS recognizes are actually the cores in the processor, and the 4 "CPUs" are due to the hyperthreading, which I understand creates a virtual core for every physical core. Is this correct? Is there a reason why different CPUs receive different usage rates at a given time? 1 and 3 seem to be the most active while the system idles, followed by 2 and 4. (Hardware)