I think it is down to the heat treatment and how close the temps were kept to optimum. There is an optimum temp for hardening and an optimum temp for the tempering back to spring hardness. This is assuming a quality steel was used. The tempering process can be anything from brittle and stiff to near mush and I've had springs that were soft and went flat as well as springs that broke in a few 1,000 shots. I've also had springs that stay at the intended performance level for 10,000 shots, but that's about the limit. I always wanted a spring made from Vasco-wear valve spring wire. I've heard of knives made from it that were almost un-sharpenable, but they held their like nothing else. In the end I just buy a few springs at a time and they are almost always just fine.