Memory usage

I am having I phone 5s with 16 gb of memory space. Right now in storage option, its showing used is 11.4 gb and available is 260 mb. I can understand for IOS, the rest 4gb is reserved. But when I checked with all other apps, I got that, all my other apps including photos, music and videos the total amount of filled storage is only 7 gb. So, I am a bit confused where is my 4.4 gb got used. Can anyone please help me on this?