purpose of -r flag for git-svn fetch

I've tried doing git-svn fetch in batches because it takes too long to
do it all at once.

    git svn -r1:5000 fetch
    git svn -r5000:10000 fetch
    git svn -r10000:15000 fetch

Strangely, after this is done if I look in the history with "git log",
I only see commits on master branch up through the 5000th revision of
the SVN repository. Someone told me to then call git-svn rebase to fix
it. What does "-r" actually do when invoked on consecutive calls to
"git-svn fetch" and why does git-svn rebase appear to fix it?

I posted this question to Stackoverflow before realizing that this
list is a better audience for it:
http://stackoverflow.com/questions/3144683/master-branch-missing-revisions-after-sequential-git-svn-fetch-calls

Thanks!