The Ford, VW and Seat people carriers all used to be manufactured from the same factory and production line, are they identical?
Conversely, the old C-Class used to manufactured by Mercedes in Germany and also South Africa, are they identical?
MB batteries maybe manufactured by Varta, but it may (and probably) manufactured to a different spec to the retail batteries. I am not saying that it is higher spec to the retail version, in fact often custom spec may be of a lower spec due to consumer law requiring specific packaging/labelling standards and certification. For example a battery installed inside a mobile phone (that cannot be removed by the user) may require different labelling and certification from similar batteries that are intended to be sold directly to consumers. This is one of many reasons why some companies chose not sell spare parts directly to the public.