SSD drives are either read intensive, write intensive or mix use. The same hardware can be used for all three, but the difference will be in the firmware, and will result in different capacity sizes. The hidden capacity is used to replace cells as cells wear out, which will occur more frequently with a write intensive drive. This is why a 1TB write intensive drive will cost more then a read intensive.

On a related note; getting exposed to some Intel Persistent Memory at work, which is non-volatile memory, which can be used as RAM or storage. It is a little slower that RAM, but a lot cheaper, therefore a hell of a lot faster then any storage, but more expensive.