You will be surprised how much data gets shifted in the background without your knowledge, Apps sniffing data, upgrade checks, cloud back ups, Camera roll baks etc the list is endless, worth checking what boxes are ticked under various apps, WiFi only etc.
One of my Sons got hit a few months back on his 4G Plan Android phone, just over 120GB in one month of chargeable mobile data, despite him swearing he hadn't downloaded anything. Took me a while to work out what was going on, turned out to be a Camera Roll back up app to his OneDrive, wasn't OneDrive itself but another 3rd party app, a Video file about 800mb in size was being backed up to his Onedrive together with his pictures, there wasn't enough space left on the cloud account so after about 500mb of uploading the video file would crash, get rejected by Onedrive, then the app would try and upload it all over again, this action was repeated all in the background without the app being active or showing on the phone, about 8 times a day 247. worked out about just over 4gb of upload per day.