Comment # 14 on bug 931724 from Martin Pluskal

Also as already suggested in comment 3, when comparing apples and apples:
# df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda3       116G   54G   61G  48% /

# btrfs filesystem usage /
Overall:
...
    Free (estimated):             60.31GiB      (min: 60.31GiB)
...

I get almost same numbers.