ipad - Available space

Hi,

I am struggling to find an answer for this question for a very long time. I have a ipad mini 16 GB. When I go to Settings > General > Usage

Storage summary says I have only 544 MB available. If I calculate all the listed apps under "Storage" under Usage section - total comes only to approx. 3 GB

I would expect my available storage to be in GB but it reports as few MB. Can you please help how to reclaim my remaining storage. Sounds like there are

unwanted temp data which needs clean-up. Not sure how to proceed?

Appreciate your help