How BB OS 10 calculates the memory status ?

I have nearly 4 GB data (i actually calculated), while my BlackBerry Virtual Expert "Device Monitor" shows me total used space is 8 GB out of total 16 GB.

When i checked file browser to measure each folder properties. I noticed my Music and Video folder are duplicated. Which basically doubles the total consuption.
when i tried to delete some data from one folder, it got deleted from the other folder as well. Means they are actually the same folder.

Can anyone help me here. Please. I can provide you screen shots, if not understood.

ABy

The OS takes a huge chunk of space. There's also app data, which doesn't appear on the file browser.
The "duplicates" are shortcuts required by the Android runtime.

Get an SD and put your files there.