Digital BW, The Print

Can someone explain why the "visual densities" of the original 
quadtone ink set (as printed on paper) are bunched at the dark end of 
the grayscale (4-45, 4-50, 4-75 are all very dark)?  Seems to me that 
if the visual densities were more evenly spreadout over the grayscale 
spectrum that we could produce more evenly and smoothly transitioned 
RGB partitions that use all the inks.  BTW, should we start to call 
these partitions B/W profiles?.

Jeff Randall