iPhone 6 Full Storage issue

Hi everyone!

I own a 16 GB iPhone 6 with the latest iOS 8.2.
My phone had been working amazingly until I started getting notifications of "Storage Almost Full" very often, until it got to the point in which I could not even take a picture nor use an app properly because apparently my phone's storage was running out. Truth to be told, I had many pictures and apps that I started deleting like crazy. However, this did not solve the problem: I had used 10 GB with 2 GB of storage left (the rest 4 I guess its for the iOS) and once I deleted MANY pics and MANY apps it got 12 GB with 0 MB available!!!
I backed up my phone, re-setted it AND restored with significantly less pictures and apps and still I have the same problem.
Basically, I keep deleting files and my free storage keeps decreasing or even fluctuating between 11 GB or 12 GB with no storage left. What could be the issue?
When I connect my phone to iTunes, it shows that most of my storage (6 GB) is taken by "Other", whtat's that?
Also, I checked on "settings" the usage tab, and it shows how I am using my storage and all the files do not add up to 11/12 GB- AT ALL. Here is a screenshot:

What is the issue? How can I solve the problem?