I purchased a MiFi from a local carrier ($20/unlimited) and decided to save money by going to the 200 MB plan. I occasionally would turn off cellular data in the phone and leave it that way for days at a time (days off work, don't leave house for more than a couple hours). I would look at my online viewer and find NO data transactions for the entire period.
I also find my phone's built-in data counter to consistently be ABOVE the usage that AT&T says that I've used.
It sounds like the iPhone or iPad is actually using data for random things, not that AT&T is charging for usage not actually used.