When creating a container by default it creates a FS with 5% EXT4 reserved space, e.g.
tune2fs -l /dev/mapper/data--ssd-vm--999--disk--0 | awk -F: '/Block count:/ {gsub(/^[ \t]+/, "", $2); total=$2} /Reserved block count:/ {gsub(/^[ \t]+/, "", $2); res=$2} END {printf "Reserved: %.2f%%\n"...