Free disk space in Finder vs Terminal df command

i am in process of writing a shell script to backup our data.

before starting the backup process, i am checking how much space is left on the disk:

terminal> df -k /my_data_directory/

Filesystem 1024-blocks Used Available Capacity Mounted on

/dev/disk0s2 244781464 173055384 71470080 71% /

this states 71.47 GB free space on /

however, the Finder reports 96.17 GB free space on Macintosh HD (which is also mounted on /).

where does that difference come from? how can i "calculate" the real free disk space?