QTR-Quadtone RIP

after defining ink limits on my paper of choice and getting a maximum density of 1.68, finding crossover points, then linearizing the curve with a 21 shade step wedge, my maximum black changes to 1.63-1.64.

Why the drop? I know it's insignificant and doesn't bother me, but why does the QTR correction curve being applied during the linearization step change maximum density?

I would think the correction curve would change all the values in between max black and paper white or does the program create its correction curve based on paper white?

Making any sense?

Jbush