I've been trying to post this for a while, but never came to it.
Here it goes.
On every Linux partition there is a certain percent (usually 5%).
For certain partition you can check the amount of reserved space like this:
sudo dumpe2fs /dev/sda3 | grep "Reserved block count"
On my computer I had about 6Gb of reserved space.
You can gain those 6Gb of space back simply by using:
sudo tune2fs -m 0 /dev/sda3
I DON'T recomend using this on you root, boot, ... partitions. More on this can be read at How to Gain Couple Of Gb Of Free Space On Linux