Hidden internal storage I've used a mobile phone with 16 GB internal storage where 4.81 GB used by OS and 10.72 GB out of remaining 11.19 GB. But when I calculate the total storage used by other features, only 8 GB (approx.) is found. Where are more than 2 GB memory used?
That space taken up when you drives are formated..it does not show the full 16 gb . What it does is partitions the 16 GB into 2 drives than it formats both...just like a PC. These are files used to make the hardware work with the software..sort of like a middle man so to speak.

For example : based on your operating system , a 16Gb drive will only read 14-15Gb after being formated.

Another example: stick an empty 16Gb SD card in and than check the free space...it won't read 16GB.
Please note: you can use any size SD card...just make sure it's formated (empty)