When mine was new, it shows one number, when it had Lion it showed another number, upgraded to Mountain Lion, it showed another number, then I loaded all the apps, and of course... ad nauseum.
The amount of free space for a new Macbook Air with a 128 GB SSD should be approximately 105 GB. There are several ways you can increase your free HDD space on your Macbook Air, including cleaning your hard drive or utilizing external storage.