iPad Mini - storage amount completely wrong?? 10GB missing??? HELP!!!

My 16GB wi-fi iPad Mini (Jan 2014) running iOS 7.0.4 is currently saying that I've "used" 12.4GB of my available storage & only have 496MB left available, which doesn't make sense because I have nowhere near 12.4GB's worth of data & apps installed on the iPad........ Ok now, I'm not completely clueless! I am fully aware that the total available storage on an iPad Mini is in reality a good 3-4GB less than the device's actual overall stated capacity (16GB in this case)due to the size of the OS itself & any pre installed software, so I'm not naively expecting to be able to install a full 16GBs worth of apps & data. I do, however, expect to be able to download & install more than 3GB worth before the "your device is almost full" pop-up appears, which brings me to the heart of the problem here......

When I manually add up the size of each of my installed apps (including the size of any stored 'documents & data') as per the size listed next to each app in the "Usage" section of "Settings", I get a total of just 2.55GB; a big difference compared to the 12.4GB that the device is saying I've used..... WHY???

OTHER RELEVANT INFORMATION

* Included in my calculation of the 2.55GB of storage that I've actually used is:

- all installed music

- all installed/shared photos & movies from the Photos app

- all installed iBook media

- apps that report having "no data"

- the listed data as used in each of the Messages, Calendars, Mail, Sounds, Safari & Contacts apps

- the specified total data usage for each app - including both it's size & 'documents & data' value

* the "Other" category that is usually listed in the 'Usage' section is not currently showing up on my iPad Mini. "Other" is not there.

* I have 34 apps listed in the "Usage" section of my device - including all pre installed apps & those which report having 'no data'.

* the following is a breakdown of the 34 apps on my device & their size (including both the app's size & that of any "documents & data") as listed under "Usage":

- Music 1024MB (1GB)

- Kindle 227MB

- Photos & Camera 151MB

- Facebook 128MB

- Evernote 94.1MB

- Notability 97.1MB

- Grafio 88.9MB

- iBooks 77.9MB

- Procreate 72.6MB

- Goodreader 71.4MB

- EBay 67.4MB

- Chrome 53.0MB

- Udemy 44.2MB

- Dropbox 38.0MB

- Sketch Book 37.7MB

- IM+ Pro 7 33.4MB

- Pocket 33.0MB

- Instapaper 31.7MB

- iDraw 26.3MB

- Weather Au 25.1MB

- Zite 22.5MB

- Flipboard 21.5MB

- Lynda.com 20.0MB

- Mail 17.0MB

- Prismatic 16.0MB

- Calculator 13.9MB

- Readability 12.4MB

- Messages 10.4MB

- MxTube 9.6MB

- Contacts 3.5MB

- Safari 3.4MB

- Calendars 1.5MB

- Sounds No Data

- Videos No Data

TOTAL = 2549.5MB or 2.49GB

**** Why does the manually added size of all my installed apps & data equal just 2549.5MB when the device itself states that 12.4GB of storage is already in use???......

**** Of the approximate 12GB of storage that should realistically be available for me to use, why is it that 9.5GB of it appears to be missing / unavailable to me???? That is 75% of my storage space that's seemingly vanished from my iPad Mini.

**** Why, when I download say a 10MB app, does 200-300MB vanish from my 'available' storage amount instead of just the 10MB of app??? Why when I delete an app over 200MB in size does only 5-10MB of space free up & not the full 200MB from the app????

WHAT'S GOING ON???......... HELP!!!!!!