why does my 16 gb iPhone 6 plus only give me 5 gb worth of memory

i just wanted to ask why i only get a little more then 5 gb of memory on my iphone on itunes to see where the memory was being used and im only using 3 gb of memory in photos and videos and 1 gb in music and 1 gb for apps and data and documents and the rest is categorized as "other" now i know as a fact the advertised GB is not the actual storage amount because some of the usage is IOS programming whic usualy take up for 4 gb that's why the amount of usage after the IOS programming the starage amount is 11 gb just like my ipad there is 5 gb unaccounted memory for i searched my iphone up and down for what is using 5 gb that is considered under "other" with no other explanation of the memory usage im tired of consistently having to manage my iphone when i cant even use 10 of the gb i paid 560 bucks for this 16gb iphone 6 plus and i only get to use 5 gb that's a BULL CRAP i got more memory out of my 8 gb IPhone so you can see why im getting mad when i phone with have the advertise memory gets get more then an a 16 gb iphone and both have the same IOS version 8.4.1

mind explaining apple