Actually, I thought this WAS the case, at least that's what I was told on a couple of occasions when going thru driver fittings. Seems the Titleist's bore-thru head designs (975D, 983K, JVS, 905R) would tend to "stiffen up" the characteristics of the shaft and the "made exclusively for Titleist" shafts accounted for that. For example, if you were to install an "off-the-shelf" Graffaloy 35"S", it would end up being closer to an "X" once assembled, whereas a "made for Titleist" Graffaloy 35 "S" truly was an "S".
That's what I heard. Could just be more marketing hype.
Robbo