One reason the measured draw weight could be less than the indicated weight would be if the limbs had already started to delaminate. One time I bought a bow, and when I got it home, I noticed that it seemed much easier to draw than it did when I test fired it before I bought it. The reason was that the top lamination was beginning to crack. I noticed it before it blew up, because I could see the crack beginning to form. The bowyer made good on it, but it's a strange, unsettling experience to go through.