Subtle differences might be caused by different production batches. Back in the day, photographers used to use the same batch stock for any given work. While the same stock from different batches will behave in the same way, mixing it would lead to small (but potentially perceptible) differences in tone and contrast.
Anyways, Kodak has dozens of film patents, many of them versions of the same film. So it’s not crazy to think that they used different versions for their own brand, very close to Proimage and identical in practice to ColorPlus for legal purposes.
Andrés PB