Mobiledata

There are phone functions that will use mobile data even when you are on WiFi such as visual voicemail and location services. Any app could also use mobile data even when on WiFi, if your settings allow it. However, those time stamps on your data usage are not real time. They reflect when data usage was reported, not necessarily when it was used. Actual usage could be from hours to days earlier. Your phone may or may not be using that data in the middle of the night.

sandblaster

Thank you very much.  Can we find out which App fetched that mobiledata?

No. ATT only records how much data is used, not what uses it. Your phone tracks data usage by app, it might help you figure out what’s using the data.

To expand on what @sandblaster wrote:   Data time and date stamped, often in 3 hour batches at midnight, 3 am and 6 am, was not used overnight.  This is data that was used when your phone was not on wifi, during the previous day, or up to 72 hours before posting in usage reports.  

    Data usage is most frequently posted to usage reports during off hours, after phones disconnect from mobile data, connect to wifi, when you're home or phone is off.