Why does Fully Utilized Thread Change over time?

While I was running a simple program which needed to run for a couple minutes, I made an observation I can’t find an answer for on google.

Here is a snip of my system monitor which demonstrates my computer switching which processor is at full capacity while running my program. my system monitor

This isn’t causing me any trouble, but I find it interesting, because I would assume that switching between units would take more time than just running at 100% capacity the whole time on one unit. Is there any simple explanation for this behavior?