CReed, my understanding is that even though the packs are fed firstly into the mixer unit, the location of the pack inlets and zone outlets allow for a certain "ballpark" temp control between front (cockpit) and rear (FWD Cabin and AFT cabin).
The schematic shown in DSC 21-10 ARCHITECTURE would seem to back this up, though of course this diagram is a simplification and may not show accurately how this rough zone temperature control is acheived.