How is it possible that my iPhone is using data when on airplane mode?
@Tyson's comment provides some insight based on your checking the data usage with your carrier, AT&amp;T.

The time that is posted by AT&amp;T is the time that they added the usage to your account, not necessarily the time that the usage was done.

As an experiment, if you can spare one of your iPhones that long, is to set it in Airplane mode for a day or two and see what AT&amp;T reports.  This thread on the AT&amp;T forum explains some of the fine print.