I had the same or, rather, the opposite problem — Usage in Settings well below that being reported by AT&T
That’s exactly the same problem the rest of us are having. It’s becoming pretty clear that the iPad is counting the exact number of bytes sent & received whereas AT&T in their user agreement says they “round up to the next full-kilobyte increment … on each data session”.
This reminds me of the time several years back when the public learned that the carriers were rounding up to the next higher minute when you placed a voice call, e.g. if your call lasted 9 minutes and 1 second you got billed for a 10 minute call. The difference here is that voice calls are well defined; it’s clear when they start and when they end. However on a computer, “data sessions” can mean many different things. Indeed I find it very ironic that in the AT&T document entitled “Session Based Wireless Data Services Agreement” the word “session” is completely undefined!
A single web page load could be interpreted as a session for example. But given how bad the discrepancy is for some I think it is pretty clear that AT&T defines a session with much finer granularity, thus giving them more opportunities to “round up”. Each little graphic, each frame, each ad, each component or object within the web page, etc. are all likely to be “sessions” with each one rounded up to the next full KB increment. In the worst case scenario, a simple TCP ACK which is only a few bytes long could be counted as 1 KB. Who knows?
The problem is likely to be exacerbated by having many shorter web interactions rather than fewer longer ones, since each separate interaction provides for greater opportunities for AT&T to “round up” “sessions”. That in turn explains why some people are off by a few hundred KB only while others (like me) are off by several tens of MBs. The nature and usages by each customer is going to be different.
Only AT&T knows for sure what a “session” means to them. They need to define their terms so that their users are aware of the true agreement for which they’re signing up.
Ray