HTC 820 internal memory full abnormal problem

Hello i have htc 820 dual and it is supposed to have 16gb internal memory. And i figured out that 9.12gb only is usable and the rest is for the system. My problem is that my internal memory is full and i figured out using sd maid that my files occupy only 1GB of that space.

So, my question is what is hogging the rest of my phone memory? does anyone know about it please? Can that be a bug that have anything to do with the recent lollipop update? thanks in advance waiting for your reply people.