Where exactly are my 16GB in iPhone?

I have bought a 16GB iPhone 6.

I am surprised how little memory I can use. I installed a couple of apps, made a dozen of pictures and all of a sudden I have less than 9GB free.

if the 7 gigs were my files then I would not mind it, but actually it's used by something else.

to be precise I went to settings And checked for storage usage. It says then that :

- I have 9 gigs available

- I "use" 3 gigs

this is adds to 12 only. So more is missing than I actually use.

but that's not the end.

Below I found a list of apps installed. I decided to add the size reported for each app and it summed up to..

1.1 GB.

So what's the catch. How is it possible that after using 1 gig out of my 16 gigs I have 9 gigs free?

Best regards,

Kamil