That is by design.  The media profile dictates how much of each color to use when printing a specific color.  Just seems like most designs have a lot of magenta and yellow components in them, so it uses more of those colors.  Even though you print a lot of black, if you print only black (K) ink, you will not be happy with that black, as it is a crappy brownish-color when printed...you have to use ALL of the inks to get a good Rich Black!