Restoring a backup key

[Osama Khalid]

After a chat on the gnupg IRC channel, I found out that the backup gpg
made was only partial. It was for one of my secret subkeys.

$ gpg --list-keys
[...]
pub   2048R/5BFA8C2E 2010-10-01
uid                  Osama Khalid (osamak) <osamak at gnu.org>
sub   1024R/567A2834 2010-10-01
sub   2048R/D023251F 2010-10-01
$ gpg sk_<...>.gpg
sec  1024R/567A2834 2010-10-01
$

The primary secret key is not included in the "sk_<...>.gpg" file. I
wonder why gpg would suggest that "backup of card key saved to
<...>/sk_<...>.gpg".

Am I missing something here?

--Osama Khalid