Internal storage problems

My phone (iPhone 5s - 16GB) tells me my pictures and camera add up to 1,9 GB, but iTunes says 3,59 GB, why is that? If I connect the phone to a computer and look at the properties of the DCIM folder, the folder only weights 1,67 GB.

Also the storage keeps going up and down without me downloading or erasing anything, with every background download disabled.

Thanks in advance