Data usage difference between iPad and AT&T

On my 3G iPad, there seems to be a difference as to my data usage.

Under 'General > Usage' it says that I've sent 6.2 MB and received 159 MB. Yet on my AT&T account, under 'Cellular Data > View Account' under "status" it says: 0 MB used / 250 MB left for 20 days.

I'm sure the former is correct, but I'm just trying to make sure. In any case, why the large, and obviously incorrect, discrepancy on my AT&T account?