It might not apply to your SD card, but I have seen very cheap USB sticks with unusual capacity and the reason was: the production could not ensure the quality of all 128 MB (yes, that was MB time), so the capacity was set to whatever could be ensured during the testing process.
Applied to your 256 GB SD card, that would mean: it is produced with 256 GB in mind, then the defective parts are crossed out, leaving you with tested and guaranteed 200 GB (+ perhaps some spare GB for self recovery).