The additional quantity of cyan required to create a neutral gray. In theory equal measures of cyan, magenta and yellow (CMY) are needed to produce neutral gray, but in practice, this additional cyan is necessary.
IF the inks were "pure" (Magenta is the problem) and reflected correctly then y-m-c equal values would work. Since they don't Cyan is always higher in the grey build. i.e. 25C/16m/16y 50C/40M/40Y 75C/63M/63Y.
