I received a message from ATT that prompted me to check my data usage. So I went to Settings->Cellular Data, and I found that my iPhone had used 22 GB of data, including 7 GB for the app store alone! My iPad also used 9.6 GB.
Panic ensues. I contacted both Apple and AT&T and asked them how it was possible that I could use 7 GB on the App Store in one month. The App Store has a maximum of 100 MB for cellular OTA downloads, so even if I downloaded only maximum-size apps, I would still have to download 70 apps to hit that amount (which I knew I hadn't done this month).
AT&T asked me if I had background updates turned on (which has nothing to do with the App Store), and they also asked me why I wasn't shutting down apps manually after using them (which you aren't supposed to do on iOS). They also tried to accuse me of turning off the 100 MB limit (which is impossible).
I ran through my recent app downloads and estimated that my total for the month should be about 500 MB, but AT&T kept insisting that, if I allow background apps and background updating (neither of which has anything to do with the App Store), then the 7 GB total for the App Store was justified.
I asked them repeatedly for an accounting of how my App Store downloads supposedly reached 7 GB, but they told me they do not maintain such records for my privacy. (I can't help but wonder what their response would be if the NSA had asked them the same thing.) I pointed out that, if they can't justify their billing, they may as well just say I use 100 exobytes, but they just continued to blame me.
I was about ready to start a petition to shut down AT&T altogether when, later that day, I did some checking and realized something that neither Apple nor AT&T had bothered to explain to me (or possibly didn't know): On iOS, if you go to Settings->Cellular Data, the usage displayed is NOT monthly usage. It is cumulative usage that generally only resets if you scroll all the way down and tap on "Reset Statistics."
The reason I received a message from AT&T regarding my data usage is that I am getting close to my monthly quota for my iPad (which is only 3 GB). After reviewing usage, I believe that that part is reasonable. However, because I received a notice simply saying I was running out of data, I checked the usage numbers for both devices, which is where I found the information that caused me to worry.
It turns out I didn't use 22 GB of cellular data in one month - I used 22 GB of cellular data in 10 months. AT&T's billing is only based on my monthly usage, and there is actually nothing wrong with my bill. I simply misunderstood the Cellular Data screen on iOS, and two tech support reps couldn't figure it out either. Apparently, the tech support people jump directly into their "whatever we say, goes" scripts without even checking the actual data usage.
I don't know what to say other than: I'm an idiot, and so is AT&T.