"Cashmere, like most other fibers except wool, does not have "memory". That means that once stretched the fibers will not return to their original shape. Thus, most of the finer fibers (angora rabbit, cashmere, alpaca, etc.) is blended with fine wool such as merino (most popular blend will be 70% finer fiber with 30% fine wool) to give it memory so sweaters etc will "bounce" back to their original shape after wearing/washing etc."
That might explain why some cashmere sweaters barely shrink in the laundry, and others shrink real small and practically felt (you can't felt cashmere, but you can felt wool).