How much you have that is available is a combination of a few factors.
First, there is the difference in terminology between what manufactures say a KB is and what a computer reads it as (or phone in this case). Manufacturers say a KB is 10^3 bytes (or 1000) a PC thinks it as 2^10 bytes (1024). Manufacturers think 1MB is 10^6 bytes (1,000,000) a PC thinks it is 2^20 bytes. GB is 10^9 vs 2^30....etc
So for a math conversion: 1,000,000,000 / (2^30) = ~.931GB Multiplied that by 16 puts you at 14.896GB which your device can see.
You then have to factor in partitioning it uses behind the scenes, specifically the OS and some other things. This takes roughly 1.5GB which drops you down to about 13.3GB of actual available space. Then the breakdown of the available space to use, is shown in your picture. The 3GB of music, 677MB of apps...etc all gets subtracted from the available space to you (13.3GB) which leaves you with the 9.39GB of free space.
Sorry if this post is confusing, drinking the first cup of coffee as I type this haha. I can clarify if it doesn't make sense, or if you have questions.