SSDs reserve extra space for over provisioning:
Over-provisioning essentially refers to “spare area” that is reserved for background operations that are fundamental to the drive’s performance. NAND natively has around 6 percent over-provisioning built in, so as SSD capacity goes up so does the corresponding percentage of over-provisioning. This increase in “spare area” results in greater performance in the larger capacity drives.
Second-to-last paragraph here:
Micron Innovations Blog » The 64GB RealSSD
Some manufacturers play it a little loose when reporting capacity. I guess their rational is that the drive
is using it, even if you
can't. I know some of the seemingly odd sizes you see (60GB, 100GB, etc) are due to that particular manufacturer being a little more forthright about it.