Storage

My iPhone shows only 2.8 gb of available storage out of 24.9 go (32 gb phone). However when I add up all bytes from used storage of all photos, music and apps I get a total of 8.5 gB. Where is all the bytes going. Why do I have only 2.8 free if I should have about 16.5 free gb based on this calculation ( I did this under settings- storage- manage storage)