There is more to it, than just specs. iRex seems to have managed a technology, which allows for waveform specifically made for certain eink batches. Those seem to result in much better contrats than standard waveforms.
Then, there seems to be some degredation of the eink screen after a few weeks of usage.
And there seem do be differences in contrast depending on the eink batch.
And there could be production steps which influence the contrast.