This is a really hard thing to diagnose.
Are you running a custom rom?
On staock rom the phone is setup to push all data when tethered through a specific APN that is different from what your phone uses for it's data.
That is how they knw how much data is being used.
It is possible that if you are running custom rom that it is sending data through the wrong one.
And I know this is going to sound to obvious, but the hotspot you make with the phone is it password protected and different from what anything else has access to?
If it is unsecured that could just be that someone else is leaching off your connection.
If neither of these is the case you would want to then find a bandwidth monitor for your computer to see if there is any other programs that are running in the background that are doing something without your knowledge.
I know I accidently has windows check and update on its own and between a office service pack and other updates it used 2gb on mine. Never knew it was doing it.
Sent from my Transformer Prime TF201 using XDA Premium 4 mobile app