Block size of my partition is 4096 (it can be checked using dumpe2fs command). So reserved space on my partition is about 6,3 Gb and equal missing size from hd command.
How to change Reserved Block Count
Reserved Block Count for given partition can be changed using tune2fs command
sudo tune2fs -m 0 /dev/sda3
where /dev/sda3 is your file system identifier
-m option sets the percentage of reserved file system blocks (in this case it is being set to 0%)
I think this point should be mentioned at the top of the post, given how many new users don’t keep separate root (/) and separate home (/home) partitions.