Ext keeps some reserved space so that if the disk is full a root user can login and still do stuff in the reserved space. However this value is quite large. If you have a modern disk then it can be many GB of space. As its default is 5% then for example on a 2TB disk that is approx 100GB of space reserved.
Normally I change it down to 2% from the default of 5%. The command would be (assuming the disk is /dev/sda2):
tune2fs -m 2 /dev/sda2
Then you can get some of the reserved space back :)
To check the current reserved space:
tune2fs -l /dev/sda2 | grep 'Reserved'
Compare the reserved block count prior and after running the change and it will be different. Unfortunately it does not display a %.
Run the commands with sudo if not root.