My iphone was supposed to have 8 GB but only 6.7 are available

When you buy any device, the user can never access all the storage on the device, you only have 6.7 GB because the operating system on your phone is taking up space, thus, using a part of your storage that you cannot use. My 32 GB iPod Touch only has about 28 GB I can use, likewise, my brand new 128 GB MacBook Air only lets me access 120 GB of storage because Mac OS X takes up some of the space.

Some for the formatting and system and other software you have.

I'm not sure if Apple still does this but there was a way GB used to be calculated that meant when you bought a drive it was always a few percentage below the advertised size.

Message was edited by: Limnos - see link by roaminggnome below

How Mac OS X and iOS report storage capacity

And this isn't just Apple products, this happens with all electronics that have user accessible storage.