Follow up with more into about this, my colleague dug around some more:
Out of curiosity I went through the documentation of the BB. My initial guess was, that there might be two revision of the board out there with different eMMC chips. This is not the case. There is only one version of the BB Green Wireless and the eMMC in charge is a “MTFC4GACAAAM-1M WT”. The corresponding datasheet mentions a smart mechanism, that disables faulty NAND blocks and replaces them with spare blocks (as long as they are available). My guess now: the boards with lower eMMC capacity are simply older or have experienced non optimal conditions (e.g. higher temperatures) during their lifespan.
The issue, Micron and Kingston do not agree on what “4GB” is. This is industry wide issue…
Kingston over the years has been more consistent, whereas Micron would tweak the flash sizes between different 2GB and 4GB modules (with at-least different part numbers…)
Other nand/emmc suppliers do the exact same thing…