While not super common, it is not uncommon to find RCBS die sets with dies made in different years. They may be replacements as you mentioned or sets put together during a time of transition at the factory, i.e. steel sizing dies to carbide sizing dies. Dies were not made in sets, but runs of individual dies and sometimes a set was put together from inventory with mixed dates. The reversed date code on the box end label would be the best evidence of when the set went into their stock inventory.