SSDs have a dedicated amount of spare area that isn't usable or recoverable by the user. 22GB does seem a bit high, but as an example, my Samsung 830 256GB SSD only has 238GB usable.
It's also possible that since this is a used SSD that one or more of the NAND chips is no longer usable. NAND can only be written to/erased a finite number of times before it no longer functions. Although this is unlikely, the possibility is there.