Well, with HDDs there was a lawsuit. That doesn't apply to SSDs - your drive actually has 256GiB (not 256GB) of flash. That particular SKU is 240GB which is 223.5GiB. So you might be wondering: what about the other 32.5GiB? This is in reserved or overprovisioned space. It's possible to have drives sold at 240GB, 250GB (232.8GiB), or 256GB (238.4GiB), all of which have 256GiB of actual flash but different levels of overprovisioning. This space is not user-accessible but that does not mean you don't use it: flash is addressed logically and is rotated for wear-leveling.
There are many reasons why the drive might have more OP. It's not uncommon with DRAM-less drives like your BX500 because DRAM-less drives have more trouble with writes and also higher write amplification (NAND wear), two things that can be mitigated with more OP. Drives with large SLC caches also sometimes have more OP, like the SX8200NP or SU800 (original), to help compensate for the fact that the SLC has to eventually be converted back to TLC. Etc.