While sailing, the mast is actually quite active along with the sail responding to the gusts and lulls encountered. When the mast is deflected, the time it takes to return to a neutral or normal state is viewed in terms of frequency response. As the percentage of carbon content increases, the frequency response becomes quicker, or more responsive in nature. Conversely, as the percentage of fiberglass content is increased as part of the structure, the frequency response becomes slower, or less responsive in nature.
Of course, this is a very generalized explanation. What it really comes down to is the structural properties of the materials in concert with the design shape. Although carbon fiber can create a very stiff shape, it's quite susceptable to damage. Fiberglass, on the other hand, is less stiff in use, but it's not as susceptable to impact damage. By mating fiberglass and carbon, one can balance the inherent properties of the two materials and benefit by the compromise. I think that the 75% carbon mast is an excellent result of this type of balance or compromise. If one was considering using freeride or wave sails, the choice is a very good one. Yet, if one was considering top of the line race oriented slalom or formula sails, the 100% carbon mast is the way to go to extract the maximum performance potential possible.