Most of these devices have a part that grips the penis, either a noose type, a flat strap, or a two-part gripper. All of these parts grab the penis behind the glans (head). The rods on the device need to be only long enough to stretch the penis from the base to the point where it grips. The erect measurement is made to the tip of the glans. This is farther out than the place where the device grips, and the difference in length is not counted. So it isn’t necessary for the rods to be as long as the erection because of the difference between the measuring point and the gripping point.
Besides that, since the stretcher is to be used while flaccid, you’d actually be comparing measurement of your stretched flaccid length and not your erect length. There have been a few threads lately about the difference between stretched flaccid and erect measurements. They are not always the same.