Maybe I missed something. Why are they making them as “perfect” spheres? Seems like a lot of wasted effort that serves no purpose other than to say look what we did.
Making them spherical gives them a predictable volume. Since they know the isotope used and its density, knowing the exact volume tells you the exact mass of the sphere.
Possibly because a bar has many factors that influence its volume: flatness of each side, parallelism of edges and sides, sharpness of each edge and corner, length of each side, etc. A "perfect" sphere has only one factor, its radius.