The assembly line workers did not necessarily take the "oldest" engine available when droping one in a car. It is possible that some of the engines that were in the far back part of the area where the assembly plant stored engines had a "shelf life" that was longer than others. Just in time inventory had not been implemented in those days and it was more important to ensure that assembly plants would not run out of engines rather than have to stop the assembly line because they ran out of a specific engine. As stated here, many Z/28's were built in the January/February time frame to get ready for the spring sales push.
There had to be stock piling of engines going on or a change in the type of engine the engine plants were building in a given month if Camaros models were not evenly assembled which they were not. For example, we also know that Pace Cars were mostly built in March and April and COPO's were mostly built in the Spring time period. This would have an effect on engine supply.