Why "-" shown under the Filesystem field when command df /dev executed?

Issue

• /dev is not a filesystem and it is sitting under /.

• It is expected with the df command on /dev, it will show the df info on / but it is not.

• Why "df /dev" is showing? '-'

[root@usmlvv1p40a2 /]# df -h /
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/rootVG-rootLV
                      992M  686M  256M  73% /
[root@usmlvv1p40a2 /]# df -h /dev
Filesystem            Size  Used Avail Use% Mounted on
-                      16G  100K   16G   1% /dev