There are offline packages, for Ubuntu and Raspberry Pi. It's a set of bundles with all the packages needed in order to install Armory on fresh OSs without the need to connect to an online package repository.
This doesn't apply to Mac nor Windows because these packages are already plug and play.
There is indeed no difference code wise between offline and online bundles. That may change in the future (lean and mean builds with no connectivity code for offline signers).