Why does my iPad air 128gb wifi cell only have actually 115gb?

Read: http://support.apple.com/kb/ts2419 and note that space is required for the operating system and pre-installed apps.

You didn't bother to read and understand the article, did you?

there is data space used in formatting the Flash drive, installation of the mobile OS, plus all of the pre-installed apps.

Plus, 1 GB= 1.073741824 GBs.

So, 128 GB= 128/1.073741824=119.21 GBs of actual starting storage space.

Subtract the amount for formatting, OS and installed apps and you are left with approx.115 GBs of useable data storage space.

It's also printed on the bottom of the box:

1 GB=1 billion bytes, actual formatted capacity less. Not like anyone understands it.