What doesn't data usage show? I was recently helping someone try to figure out were the data usage was coming from since recently it has sky rocketed from under 8GB to well over 30GB. Anyway, they went to the data usage under setting (I believe the phone was a Galaxy S 3 or 4) the total data usage essentially agreed with the carriers usage but the apps below did not add up anywhere close to what was said. This was over the phone so maybe I missed something.

Anyway, is there anything that is not shown by the data usage that comes with the Galaxy S III (or any android devices)? 

Is there a way to get that info, maybe with a different app?
Usually it won't happen that the total data usage is inconsistent with each app's data usage. However, you may try installing some of the data usage monitor apps on PlayStore:


My Data Usage
Data Usage
Onvao Count