On a MacBook Air (M1, 2020) running macOS Big Sur 11.7.10, Activity Monitor showed something had been going on. (The thread title is misleading. I would have said something like “Somewhat busy CPU became notably less busy only by restarting Activity Monitor” but I was trying to avoid such a long title.)
I had just unlocked the screen (see the change in activity at the right side of Cores 1-3) and saw the continued use of cycles on Cores 5-7. The main Activity Monitor window did not seem to report anything that would explain the graphs for Cores 5-7, and Activity Monitor itself was using a single digit percentage. I quit Activity Monitor and restarted it, and going forward Cores 5-7 looked like Core 8 in the image. The main window seemed just as before, with just a couple processes in low double digits.
What would explain the activity before I unlocked the screen, the lack of reported activity in the main Activity Monitor window, and the decreased activity after I restarted Activity Monitor? This seems weird.