iPad Gen. 5 storage mystery

My 32GB iPad ran out of storage. First I tried offloading everything I could imagine that I don't use and restarted it. Still full. Then I reset it to original settings without transferring apps, etc., from iCloud or my phone. There is literally nothing on it except what comes with a new iPad out of the box. Immediately after resetting it is showing: "31.9 GB of 32 GB Used.