Suggest a narrow steel tape be used for any true measurement. If wanting to face a really true measurement of the arms (or anything else) take a measurement the first thing in the morning, while in bed (lying/sitting). Don't brush the teeth, or do anything else. Experiences BB'ers can get a bit of a pump just by teeth brushing, walking around, fixing breakfast...or stroking whatever. Bottom line: you want a true measurement...as hard as that might be for some guys...not a false pump reading..
Cloth tends to stretch out or been knows to shrink in size a bit at times. Plastic can not be all that dependable either. If using those style tapes, might measure them against a yard stick, to check the true measurement..
If wanting to get a ultimate TRUE arm measurement, than take a couple of weeks off from working out. Than measure the arm. Shocking, I know, of the results some may find. Not recommended for over sensitive.
The shown video.......Bogus. Pumping out high reps with a pair of 15's off camera.....who knows.
Good Luck