A mix design is the recipe for the concrete: the proportions of cement, coarse and fine aggregate, water, and admixtures. It is engineered to hit a required strength and durability, not just to fill a mold.
Precast plants often run several mix designs for different products and conditions, tuned for early strength so pieces can be stripped and turned around quickly. The mix that goes into a piece is part of its record, because strength, cover, and durability all trace back to it when a QC question comes up.