Because of many factors.. Necks may be hand finished, therefore variables are to be expected, one may be earlier production that may have had specs changed slightly in later manufacturing, where the wood actually was on the tree would affect hardness, too many to mention.. Besides, where is the fun in having 2 identical guitars??
I had someone in US who sent me these guitars and the red one was supposed to be taken by my friend but he had some issues and could not buy it from me!
and I was playing the yellow one all the time. all of a sudden I taught I should give the red one a try and that was when I was shocked to death when I played that super super slim neck!!