I would hypothesise that the husks of grains such as wheat and barley have a higher cellulose content due to the fact that they grow a pericarp around the seed in much the same manner that they grow plant cell walls. However, trees grow nuts as a berry whose outer layer hardens. Perhaps this berry growth is the reason the hemi-cellulose content is so high as the shell of the nut is "fruit" in its first stages of growth.