There are probably a few apps on the device that are hogging data. Unfortunately many try to do things, without you knowing it, on them.
Have you tried restricting background data? Not something people commonly try, but I myself got tired of many programs running stuff in the background and running up my data usage. I found that turning this option on, didn't limit my phone really at all. Granted, I don't do many things on the device that aren't being done on WiFi when it comes to downloading, however it might be an option you can try.
It will also help you narrow down which apps are hogging your data. Any app that acts up once you put on the restriction is obviously doing things in the background and using the data.
How to Restrict...