I have been studying this issue.
A couple of questions...
At night when you reset your statistics, are any apps running on your iPhone... that is, do you shut-down all running apps before you re-set cell-data statistics? I'm not trying to be rude - but, just to verify... do you know how to shut-down apps (double click the "Home button", etc.)?
I also use ATT. When I look at my cell-data usage at:
the usage data is about 2 hours behind real time. How many hours is your ATT detail data-usage behind real-time?
When you say "turning the phone off" are you putting the iPhone to sleep by briefly pushing the power-button or powering-off the iPhone by holding the power-button down for 5 seconds?
But, anyway... to answer your question... If the ATT detail cell-data usage report says your phone used 5040KB at 1:46am I would think you would have to see that on your iPhone cell-data usage screen.