Memory settings menu inconsistently shows internal storage half-empty and completely full at the same time
Devices with internal SD cards / memory (for instance, my Galaxy S3 has 16GB), are split into many partitions.

4GB goes for OS/bootloader/recovery/system/etc

12GB is partitioned into /dev/mmcblk0p15 and mounted on /data

/data contains 4 main folders:


/data/app
/data/data
/data/dalvik-cache
/data/media


/sdcard (aka your user-available storage) is not a real partition, its actually a 'symlink' of sorts to /data/media

So the settings->storage (and a lot of apps, eg; ES File Manager), will report 12GB as your "total" partition size for /sdcard, even though your "sdcard" actually has 12GB - app - data - cache = 10.5GB (in my case) space available to it.

You need to be rooted, and use an app called "DiskUsage" from google play.. It will show you where your used space is going. Hope that helps!