More evidence AT&T's meter is rigged. From DSL Reports:
After finding that AT&T can't add its own stats correctly, I decided that more in-depth checking was warranted. Luckily, my DD-WRT router is the last stop before my DSL modem, and it automatically keeps upload and download usage meters. I already had everything necessary to fact-check AT&T's usage numbers. When I compared the daily AT&T usage data vs DD-WRT's usage data, I was in for a shock. It appears as though AT&T's data is entirely corrupt. Not only was I being wildly overcharged on some days (to the tune of a 15GB overcharge in a single day), but I was also being undercharged on other days.
The errors become glaringly obvious when the AT&T reported data minus the DD-WRT reported data is graphed. Points above the axis are overcharges, points below the axis are undercharges:"
