I think it's because of the rate of acceleration their arrows go through versus ours, if they shot the kind of tip weight we do, their arrows could break. We always think the amount of spine we need is derived from the poundage and draw length we have, which it kind of is but it has more to do with how fast our arrows accelerate. I just switched from pretty fast recurve to a really fast recurve and dropped about 5 pounds in draw weight, but my speed went up about 5fps due to the bows efficency and I was able to use the same arrow. Where had I simply dropped 5 pounds of draw weight and stayed with same efficency bow, I probably would have had to go to a weaker arrow. At least this is the way I see it, if I'm wrong, somebody correct me please.