Android Internal Storage is not completely empty?

[AC Question]

I have a Sony Xperia Z (the first one) and I just did a Factory Data Reset (I also chose to erase internal storage) but the Internal Storage is not completely empty. It has a full capacity of 11.7 GB, but only 10.0 GB are free. Why is it not completely empty (11.7 GB free)? What's taking that space? Is there something wrong?

 

[dpham00]

That's normal. The os takes up some space.