git push on tracking branches

Hi all-

A coworker alerted me to some strange behavior with git push on tracking branches (maybe a documentation error?).  Pro Git (http://progit.org/book/ch3-5.html) says:

"To set up a local branch with a different name than the remote branch, you can easily use the first version with a different local branch name:
   $ git checkout -b sf origin/serverfix
   Branch sf set up to track remote branch refs/remotes/origin/serverfix.
   Switched to a new branch "sf"
Now, your local branch sf will automatically push to and pull from origin/serverfix."

When I do this on my local machine (current master on Mac 10.6.4):

snowg5-2:temp stephen$ git clone ssh://bash@penguin/home/git/repos/test-repo.git
Cloning into test-repo...
warning: templates not found /Users/stephen/share/git-core/templates
remote: Counting objects: 100109, done.
remote: Compressing objects: 100% (23394/23394), done.
remote: Total 100109 (delta 76733), reused 99567 (delta 76231)
Receiving objects: 100% (100109/100109), 620.24 MiB | 28.36 MiB/s, done.
Resolving deltas: 100% (76733/76733), done.
snowg5-2:temp stephen$ cd test-repo
snowg5-2:test-repo stephen$ git checkout -b tmp origin/real-branch-name
Branch tmp set up to track remote branch real-branch-name from origin.
Switched to a new branch 'tmp'
... edit some files ...
snowg5-2:test-repo stephen$ git add -u .
snowg5-2:test-repo stephen$ git commit -m "made some changes"
[tmp 0440ace] made some changes
 1 files changed, 1 insertions(+), 0 deletions(-)
snowg5-2:test-repo stephen$ git push
Everything up-to-date
snowg5-2:test-repo stephen$ git rev-parse tmp
0440ace51b4ab18eee39305cd2af070f572e38d7
snowg5-2:test-repo stephen$ git ls-remote origin real-branch-name
92ebff3a7c332079bcbf84e9cf699ab635e6ba3c	refs/heads/real-branch-name
snowg5-2:test-repo stephen$ git --version
git version 1.7.3.2.g9027fa
snowg5-2:test-repo stephen$ 

Git doesn't push the change.  If I either use 
  a) git checkout --track origin/real-branch-name 
or
  b) git checkout -b real-branch-name origin/real-branch-name
the push succeeds.

Was the behavior of git push intentionally changed or is this a bug?

Thanks, Stephen