OCZ says...
*Consumers may see a discrepancy between reported capacity and actual capacity; the storage industry standard is to display capacity in decimal. However, the operating system usually calculates capacity in binary format, causing traditional HDD and SSD to show a lower capacity in Windows. In the case of SSDs, some of the capacity is reserved for formatting and redundancy for wear leveling. These reserved areas on an SSD may occupy up to 5% of the drive’s storage capacity. On the Vertex EX Series the naming convention reflects this and the 60 is equivalent to the 64GB and 120 is equivalent to 128GB.