MY PHONE SAYS 16 GB BUT ONLY HAS 11 GB AVAILABLE

where does the other couple of gigabytes go? If not toward our music or social media or basically our own use? Why not just sell the phone true to the memory, I mean i have 300 songs and 400 photos and a couple social media apps like snapchat Facebook Twitter and tumblr and my phone can't take another photo because all my memory is wasted and I check and it only gives me 11.3 GB available for my use and I used it all up like hello if you're gonna sell me a 16 GB iPhone I want ALL 16 GIGS not 11, so check that out apple