I couldn't find info on SSD yields, do you know where to find it? I found that HDD factories generally target 97+% yield. So, if SSD yields are much lower, you bring up a good point.
http://semimd.com/blog/2014/01/29/3d-nand-to-10-nm-and-beyon...
http://www.theregister.co.uk/2015/04/09/3d_nand_gigantically...
I'm making the assumption that NAND flash is binned and derated based on defects. ie, to make this drive, Samsung needs to fab 256Gbit dies. As this is on the leading-edge of their production capabilities, I'm guessing that the defect rate is not negligible, and that they accidentally make lower-capacity chips a significant portion of the time.
They can even sell them as lower capacity drives. I'd be surprised is the yield was not 100%.