Question about "git pull --rebase"

hello,

Let's say I'm on a branch called 'foo'.

I tried to rebase this branch by using 'git pull --rebase'.

I first tried the following command:

    $ git pull --rebase origin master:foo
    remote: Counting objects: 5, done.
    remote: Total 3 (delta 0), reused 0 (delta 0)
    Unpacking objects: 100% (3/3), done.
    From /dev/shm/git/A
    ! [rejected]        master     -> foo  (non fast forward)

which failed.

Then I tried:

    $ git pull --rebase origin master

which worked.

Reading the man git-pull I would assume the 2 commands are equivalent
but obviously they're not.

So the question is: why ?

Thanks
-- 
Francis