Why Is Documents & Sync Using All My Data?

I have had enough of this issue now. I have turned off all my mobile data for non-essential apps, followed all the recommendations to sign out of iCloud and my Apple account on my iPhone 7 Plus and STILL this confounded "Documents & Sync" is using up all my data. ALL OF IT!

I am furious about this because I am now no longer in a position where I can trust this device to operate the way it is supposed to operate. My wife has the same phone and uses the same carrier on the same Apple account as me and has NO issues like this at all. It is inexplicable and inexcusable from Apple.

Who at this company actually cares enough about customer service to sort this out and give me a reason to carry on using Apple products? Because right now I am only a few hairs away from swearing off this company as my technology partner forever. I own a lot of Apple stuff and have never experienced anything as infernally stupid as this.