Digital BW, The Print

I went to the MIS website to check out the ink information... the 4-
ink separation makes complete sense: 25%, 50%, 75%, and 100%

You just divide 100 by 4 and you get the different dilutions.

However, when you go to 6-color printing, the separations are at 15%, 
25%, 45%, 50%, 75%, and 100%

Is this because there is a logarithmic response to how much ink you 
lay down in the highlights? If so, why is the 4-ink formulation using 
25% for the highlights and not something a little off 25%?

If the response was as linear in the 6-ink as is apparent with the 4-
ink setup, you would get 16 2/3%, 33 1/3%, 50%, 66 2/3%, 83 1/3%, and 
100%

Please don't slam me if the answer is obvious... I'm a noob :-)