iPhone 5s 16GB what is actual user available memory?

This is my first iPhone 52 16GB model, I am at lost as far what is user available memory; I attached it to iTunes out of the box and here is the break down:

Total available memory is 12.9GB > so where did 2.1GB go?

2.43 GB used for 16 App's, almost all Apple pre-installed, since I installed few size in MB.

1.93 GB used by "Other,: I am guessing it may be the IOS.

8.54GB available for me.

So the main question is if you ad-up numbers 8.54 + 1.93 + 2.43 = 12.9 GB, where in the world did 2.1GB evaporated.

I am likely to return this phone and get 32GB model, I wish Apple is more forthcoming on the exact space available for user on iPhones and iPads.

Thanks,

Misha