Storage on Sony Xperia Z2

Hey guys,
I have a Sony Xperia Z2, I know, it's an old phone, but it still works fine.
I have a question: if I have moved most the apps installed in my Sony to the mini SD card, just like all my pictures, music and videos, why does the phone still have a few memory/storage space? The Z2 I have has 16GB of memory, and for some reason, it only has 1GB of free space. What is causing/making this lack of free memory available?
Is there something I can do?
I look forward to hearing from you.

from Android Authority Forums http://ift.tt/2kpoWV3
via IFTTT