Why is it marketed and sold as a 128GB SSD when it's actually only 119 GB?
Copied from http://www.tomshardware.com/forum/271049-32-500gb-hard-drive-shows-space-comp .
1GB according to the disk manufacturers is equal to 1,000,000,000 bytes. According to how the operating system (and the rest of the computing world) calculates it, it's equal to 1,024 megabytes, which in turn are equal to 1,024 kilobytes, which are themselves equal to 1,024 bytes. So one GB as per the disk manufacturer is really 1,000,000,000 / (1,024 x 1,024 x 1,024) = .931GB as measured by everybody else. So a "500GB" disk has 500 x .931GB of storage, or 465GB.
This isn't because your system isn't showing some available storage or because the formatting lost storage - it's just the byproduct of the disk manufacturers exaggerating the storage capacity. It's similar to stereo manufacturers overstating the wattage of their systems, tv manufacturers overstating the dimensions of their tv's, homebuilders overstating the square footage of houses, etc.