There could a few reasons. The first thing you can do is get some digital calipers for accuracy and simply compare the two necks dimensionally. If that is pretty close, then you start looking at things like string gage, string height, neck relief (truss rod adjustment)....general set up stuff playing process of elimination.
Because they are different. I have a few 'moth necks and they are each unique in undefinable ways. Wood is like that! Plus, I think there is a psych. difference in seeing the thickness of the rosewood over the maple vs. all maple.