SL has extra packages (it means they wasn't included into RHEL) that was included in it.
... we have added several packages to Scientific Linux that are not found anywhere in the upstream release, including IceWM, OpenAFS, Revisor, Live USB Creator, YUM auto-update, external YUM repositories... (c) from http://distrowatch.com/?newsid=06549
SL and CentOS like two people that go different ways, but have the common base.
Kind regards,
Alexander Ustinov