How is it possible that my iPhone is using data when on airplane mode?

All our phones are using data during the night when we are asleep. I thought that apps were only supposed to update on WIFI. But my phone is always on airplane mode at night unless I forget, so last night I put in on airplane mode, took a screenshot as evidence, and sure enough, a tiny bit of data was used at 6:00am this morning before I got up. How is that possible?