iPad mini available storage says 0, using only 5 gb

I Have a second generation, 32 GB iPad mini. It's been getting slower and close to full for some time so I deleted a lot of photos and videos and unused apps, backed it up and then did a back to factory settings and restored it from the backup.

so, when wiped it it had about 2GB of free space. When I resorted it it now says it has 27.1 gb and no available space. But it has only restored maybe half of the apps and under usage it only adds up to about 5 GB. Any idea