Well you know since I don’t have the machinery I am going to have to rely on my anatomy training to answer this one. (The link that I provided only instructs urologists to stretch the penis from base to tip. )
Your penis is made up three cylinders that make up the shaft. The tunica spongiosum is the cylinder that contains the urethra and unlike the two cylinders flanking it, it doesn’t have the same collagen structure. This means that when flaccid, it stretches but does not get rigid. The reason why the penis is stretched while flaccid and not erect is because of the whole growers vs. show-ers phenomenon. That gives urologists a more accurate measurement.