Generally it is in the wall thickness. It can be more concentrated in one area or another. If you are comparing a 75g S flex and a 95g S flex, for example, the weight may be distributed slightly different in the heavier shaft so that the bend profile can be in the same S flex range as lighter version. A key factor in controlling flex is wall thickness and weight. If you just made the wall thickness stiffer consistently throughout the entire shaft of the heavier shaft, it really can’t play the same or have the same exact flex dynamic or profile as the lighter shaft. So within the same model where the weight is the only difference, wall thickness and weight distribution has to vary some, as well as geometry and taper rate, for the flex value to stay within the same range. Can’t tell you on any given model what the difference or where the weight might be distributed to make a 95 S flex have the same bend profile as the 75 S flex in the same model.
Britt Lindsey