When a mast is deflected it stores energy. When deflection force is removed force will be used to return mast back to its original (and beyond if poss) position.
Two forces are resisting the motion to return.
A) The weight (mass) of sail and mast
b) The dampening effect of sail
If you reduce the weight the thing will fly back quicker.(ie the force stored is acting on less weight) since Force = Mass x Acceleration: if you half the weight you will double the acceleration (back to norm) On top of this 100% masts are not only lighter I suspect they are also stiffer;(regardles of IMCS) compare a 490 100% and 490 75%. I have hung weights from them.The 100%C is always stiffer(and lighter))) so they store even more enegy (IMO) compounding the effect.
In archery a carbon bow will match (in speed of arrow terms/length of flight) a fibreglass one with only 70% of its draw weight.Because it has a faster reflex.It is not just a sail /mast term its physics/materials concept.
I also think (not sure of this one though;cant quite remember it from uni days) that carbon fibre has very low hysterisis.(ie very low internal daming;some materials show resilience to returning (polymers in MTB suspenion) some show very little (Spring steel/carbon fibre)