Draw length is a moving target. People who attend Rod Jenkins' course commonly report increases in draw length of 1/2” or more from learning more efficient ways of drawing the bow.
Draw length is variable, depending on:
-Weight of the bow. A heavier weight bow compresses the joints more in the arms and shoulders. Plus, people just don't draw a heavier weight bow as far, once weight exceeds a certain level.
-Degree of string arm rotation.
-Degree of shoulder rotation that pushes the bow arm forward.
-How open or closed the shoulder alignment is at full draw.
We measure draw length to the back of the bow for convenience. But the actual definition of draw length is measured to a spot that is 1 3/4” in back of the pivot point of the riser. I have bows where the back of the bow is 1/2” in either direction away from that spot, so going to extremes, my draw length would measure 1” different on each of those bows if measured to the back of the respective bows.
Edit: this could also explain why measured draw weight might be different from marked draw weight on some bows. If the bowyer was measuring the weight correctly to a point 1 3/4” in back of the pivot point, and you're measuring it to the back of the bow, the two weight measurements will necessarily be different if those two points do not correspond.