I'm with 3 on this.
You take TWO measurements: One with the arm relaxed. THEN, flex hard and take your 2nd measurement. The difference in size between relaxed and flexed is ONE way (of many) to guage your
muscle mass.
If there is very little difference between the numbers of your relaxed and flexed pose, then you (generally speaking) probably don't have much muscle mass (other than what you "see" cold), and OF the muscle you DO have, the muscle is soft, probably "marbled" with intramuscular fat striation,....flabby muscle, un-toned, un-conditioned.