On 10.6, when I ran Windows in a virtual machine while it was doing windows update, and left it running in the background for a few minutes and returned to it, sometimes I would get 60fps in the machine for a few seconds and it updated very very fast, and then it went back to 15fps (seemingly getting more CPU power.)
On 10.7 (if I recall) if I left Activity Monitor running in the background for a few minutes and returned to it, I would get nonsensical values for the CPU usage for a few seconds (e.g. 398423.34%) but then it would return to normal. Perhaps this error has been accumulating over the years.