Well having worked in the plastic industry and still working in the chemical/polymer industry I can answer with my limited knowledge (which isn't more than anyone here). Depending on the parts, the manufacturing process might differ as the mechanical properties (stock vs rail panels) required for the items might differ quite a lot. The actual recipe might be different as well. And there is always the issue of the color particles provided by suppliers which can also vary quite a bit.
And I haven't touched the QC issue...