iPad iOS 8 low free space on iPad

[Thomasxxxxx]

I have 16 GB iPad mini. After upgrading to iOS 8.1 free space on my iPad rapidly went down. It says that capacity of my iPad is 11.7GB (on iOS 7 I had 12.8GB I suppose). That is not problem. I understand that iOS 8 could take 1 GB more. But when I count capacity of all installed apps that is shown in statistics in settings, I got 8.4 GB. And free space is 0 bytes. And when I delete someting after some time it goes down back to 0 bytes. I don't think that usable storage of iPad should be 8.4 GB on 16 GB model. Do you have any explanation? Thank you for your answers.