ext2 ant ext3 file systems has reserved 5% disk space in case of dissapeared free space, for system proccesses. That system wouldn’t crash and would keep going.
For big filesystems those 5% are too much(with 1TB it would be 50G for system procceses), so we can tune a little bit.
You can check how much reserved space partition has with (You need root privileges for all actions):

$ sudo tune2fs -l /dev/sda1 | grep Reserved 
Reserved block count:     12441
Reserved GDT blocks:      256
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)

to change reserved space (in percents)

$ sudo tune2fs -m 1 /dev/sda1

check how much it got bigger

df -h