I reset my data usage in Settings (iPhone 4s) at the start of every billing cycle. Every month I near my data limit of 2GB so it's important to me to be careful of this number. However it seems like every day myAT&T claims my data usage went up by something in the neighborhood of 10 to 30 MB more than the usage statistics in Settings say. Now I'm pretty sure Settings knows how much data is transferred. Why does AT&T inflate this, is there any way to get them to stop, and does any other provider not do this?
The data counters on all phones are approximate, and they're most likely not artificially inflating your data usage. In the future, you can check AT&T's website to track the exact amount of data you'll be billed for.