Discrepancies in available capacity of Macbook Air SSD
It is interesting to note that Hard Drive Manufacturers use the correct meaning of "Gigabytes" when referring to hard drives, and not what most people think of as a GB which is actually a Gibibyte:

Gigabyte = 10 to the 9 bytes = 1000000000bytes = 1000 megabytes

Gibibyte = 2 to the 30 bytes = 1073741824bytes = 1024 mebibytes


Since Mac OS X also switched to this "proper" definition for storage units such as Gigabytes as of Snow Leopard, then the size of the drive should be understood in these terms, noting that on a Mac the OS and the Hard Drive will be talking the same language, there is no need to do the conversion from Gibibytes to Gigabytes.

The given size should be correct and accurate as reported on a Mac as they have a common understanding of what a Gigabyte is, and incorrect on Windows which uses the commonly misused Gibibyte structure on it's filesystems whilst users incorrectly assume it is measured in Gigabytes.

Technically manufacturers should use GiB not GB when marketting and packaging their drives, but they don't/won't, and use Gigabyte which they know practically everyone uses incorrectly.

In your case the available unformatted capacity is indeed low even for using the correct maths, but as referenced in another answer the size of 256Gb refers to the total storage capacity of the drive, and you have to not only accept that formatted capacity is lower, but also that the capacity that is available to format can also be lower, if portions are reserved for the purposes of replacing bad blocks through the lifecycle of the drive.



Lengthy comment follows after the first 2 comments below - too long for a comment tho, but not really part of the answer other than background info perhaps



The space reservation thing is not something I feel I can be canonical about, I know the theory, I can't really backup it up once and for all, especially because this is an Apple non-consumer drive.  However, almost all drives over provision, providing more space than they state in order to have capacity for replacing worn areas.

It's not 100% clear, but it certainly appears to be the case, that in the case of these Apple supplied drives, the provision of "backup" blocks is taken from the stated capacity from the off.  I think this is how most manufacturers do it, hence the fact that you see way more 60Gb drives than 64Gb etc.  The implication being that there is 64Gb of storage capacity, and 60 of it us user addressable.  Whether you call that a 60 or a 64Gb drive could potentially be taken either way.  I would call it a 60, but from a legal perspective I would imagine both cases are arguable accurate.  As for the instances you see where the available capacity is equal to or greater than the nominal capacity (in the case of the 128Gb drives you see) then it could simply be the case that some manufacturers are either more generous, or more worried about failure, than others and genuinely put in more chips worth, say 132Gb for example.

So in you case, you likley have a 256Gb drive in terms of chips, the fact that the user addressable space is lower, even before formatting is likely just how it is.  I have an Apple supplied 128Gb Air SSD which shows much the same behaviour:

APPLE SSD TS128C:

Capacity: 121.33 GB (121,332,826,112 bytes)
  Model:    APPLE SSD TS128C