Why does my iPhone say it doesn't have memory when I haven't used all of it?

I've had an iPhone 4s for about 5 months now and for some reason it says I don't have any memory left. It's got 16gb. I've used about 5gigs of music and mabey 2 in games but it says I only have 100 megabytes left. I went to system, usage and added up all the things i have downloaded and it comes up to around 10 gigabytes. Why does it say I don't have memory left?

The ios take 2-4 gb… Pictures, apps, music, updates, cookies… Take the rest.

  • Why does my iphone say I have used so much storage? It say I have used 5.3 GB. It say I have 821mb left. I added up all the apps and and its like 3300 MB. I looked it up and its like 1000 MB in a GB. So that's like 2 GB that's being used that isn't there. What is taking up all my storage. Can someone please explain?! And can someone tell me how to free up some storage if I can
  • Why does my iPhone say I have 11 emails and I have none? How do I get rid of that. When I get a new message it says 12 then when I delete goes back to 11.It's annoying. How to I get it back to 0?
  • Why does my Iphone 5s memory say other? Okay so I updated my Iphone 5s to IOS 8 and since then when I connect it to my laptop, in the memory usage on Itunes it says "other". What is that and how can I delete it/ lower the space it takes?
  • Why does my iPhone 5 say memory full? I bought an iPhone 5 In January and it keeps saying memory full even though it says I have 3.9gb/5gb left. It's a 16gb phone with only 10 apps and no music whatsoever. I do have about 3000 photographs. Even though I've deleted loads and all my photo stream plus multiple apps it keeps saying its full!