All modern SSDs set aside a certain amount of space called "overprovisioning". So even though there may technically be 256GB of flash memory, 5-7GB of it is inaccessible to the operating system. The SSD's controller moves data around in its spare time and uses that space when it needs to. The reason it does is that SSDs can only be written to a finite number of times (about 5000) before they wear out. If the controller detects that one SSD cell gets written to a lot, and another one doesn't get written to at all, it will move the data around so that the drive wears out evenly.
I'm grossly oversimplifying it, but that's what's going on. It helps ensure that the drive lasts as long as your computer.
Since the 256GB drive starts with 250GB, and OS X and the preinstalled applications take up 14GB, that means that out of the box the 256GB models have 236GB free.