QTR-Quadtone RIP

I have followed the advice I received to produce a warm and a cool
curve and then blend them to my desired tonality. I have a very linear
warm curve and a very linear cool curve. I am using both curves to
print and blending 60% warm to 40% cool. The resulting gray ramp is
not evenly separated. There is a distinct 'knee' at about 70% and a
second distinct 'knee' at about 25%.

So my questions are: 
   Is this normal? 
   Is there a way to correct it?

Thanks for all the help.

Scott