I'm sorry, but this doesn't make sense to me. I'm not sure what you mean. If you mark the serving on each side of the rail w/ either whiteout or magic marker, you can see if it's centered after you cock it. then, you adjust the centering by pulling back on string and move it which ever way you need to get the marks equal to edges of the rail. A rope cocker does this for you. If your question is why doesn't the string center itself from limb pressure once it's cocked, you won't find any xbow that does this w/o manual intervention. good luck!