Rephrasing: The packs put air into a common manifold, which is then distributed into the different cabin zones.
If one zone is particularly hot, perhaps due to lots of people in it, the packs will adjust their output temperature downwards to keep that zone cool.
However, since the packs are only outputting one particular temperature, the other zones (which are fed by the common manifold) will be over-cooled. This is why hot trim air is added to these other zones (Trim air is added in precise quantities to individual zones).