Discrepancy between Ubuntu 16.10 System Monitor % CPU and ps pcpu, % CPU utilization with respect to the individual Firefox browser process

What is the reason for the following discrepancy between Ubuntu Linux 16.10 System Monitor % CPU utilization and ps pcpu, % CPU utilization with respect to the individual Firefox browser process? For the Firefox browser app , I get 2.8 % pcpu from ps -eo pid,rss,c,pcpu,cmd while for the same Firefox browser app, the GUI Application, Ubuntu System Monitor, show 0 % CPU utilization. May I ask why this difference? An Unix Linux Stack Exchange expert's , @DopeGhoti, hypothesis about this difference is: it could be that ps is reporting actual CPU usage at that moment, but the GUI application could be showing aggregate usage over the last n seconds, or possibly rolling in CPU usage from children or iowait usage. Could someone expand on this hypothesis? Any help is greatly appreciated.