IPhone: settings cell usage vs AT&T's data usage number?

AT&T has a number, by calling it will send you a text message telling you how much usage you have used on your data. But also in the iPhone there's a cellular usage count in the settings app which also tells how much data you have used. Both of my cellular usage counts are different. Which is right?

If they are relatively close, it's no problem, one's probably delayed in updating.

If they're quite far off, then perhaps it's including Wi-Fi usage.