This is by design: Equal quantities of C,Y,M will produce a shade of gray. So (50,50,60,0) is the same color as (0,0,10,50) The color in this example is a yellowish gray. This technique is called undercolor removal.

This technique saves on toner use and is gentler on the equipment. (If your fuser has the slightest problem, a 400% black print will reveal it.)

Thank you