You are comparing the assembly date of the engine, as is mechanicaly stamped into the block, to the body build date shown on the Fisher Body trim tag. That means the engine assembly stamp was on average about 2 weeks before the body began assembly on a Z/28 that was produced during that time frame where your car was built. You will find some cars with dates closer and some that are farther apart because there was no "first in-first out" type inventory control. They were basicaly building components as fast as they were selling the cars at that time so there was not a lot of extra stuff laying around.
-Mark.