Solid State Drive (SSD) storage differs from the previous hard disk drive (HDD) mechanism in that SSD has no moving parts. Instead of using a rotating disk to store information, the SSD uses Solid State memory. When using Solid State memory, buffer space is allocated to allow all the memory locations to be used evenly. This allocation gives SSDs a longer life.
As with HDD, the SSD also allocates space to handle administrative functions such as dealing with bad locations. You can see this difference if you look at how your computer summarizes the capacity of the computer’s storage drive, for example, in Disk Utility's Total Capacity listing.
The storage drive in your Apple product, like all storage drives, uses some capacity for formatting so there will be less capacity available for applications. In addition, other factors, such as pre-installed systems or other software and media, also uses part of the available storage capacity on the drive.
You can also refer to the capacity calculation difference between drive manufacturers and Mac OS X as described in this article.