Just to chip in: it was stated that one single black model measured 'worse' than one single brown model.
Firstly, those measurements have now disappeared.
And secondly, even in Sennheisers flagship HD800, almost all the frequency response graphs they send with each model are slightly different.
It's clear to me that despite higher standards and technologies in manufacturing processes today, it's still very difficult for companies to make exact clones.
The same slight variations are found in all niche high end products that are dealing with mechanical variables and tuning from SLR cameras and lenses to cars, etc etc.