The GPU usage line in the F3 menu is inacurate

i was playing my world then it started lagging. Then to troubleshoot the failure i opened the F3 menu. It said that my graphics processing unit usage was so high that i had to turn down my render distance a bit, then i opened taskmanager.exe to look at my graphics processing unit temps to see if they were too high. Nothing was wrong, the temperature was fine. it wasnt even using all of my graphics processing unit