[CentOS] priorities don't prevent other repos overwritting packages?

Either I've made a mistake in my configuration or else packages from
other repos can obsolete core packages

For example, I did a "yum update" on some rpmforge packages and saw this:

Installing:
 perl-Date-Manip          noarch  5.54-2.el5.rf                 rpmforge  210 k
     replacing  perl-DateManip.noarch 5.44-1.2.1

 perl-IO-Compress         noarch  2.024-1.el5.rf                rpmforge  242 k
     replacing  perl-Compress-Zlib.i386 1.42-1.fc6


If I try to install the original packages:

% yum install perl-Compress-Zlib
Loaded plugins: priorities
476 packages excluded due to repository priority protections
Setting up Install Process
Package perl-Compress-Zlib is obsoleted by perl-IO-Compress, trying to install perl-IO-Compress-2.024-1.el5.rf.noarch instead
Package perl-IO-Compress-2.024-1.el5.rf.noarch already installed and latest version
Nothing to do

% yum install perl-DateManip
Loaded plugins: priorities
476 packages excluded due to repository priority protections
Setting up Install Process
Package perl-DateManip is obsoleted by perl-Date-Manip, trying to install perl-Date-Manip-5.54-2.el5.rf.noarch instead
Package perl-Date-Manip-5.54-2.el5.rf.noarch already installed and latest version
Nothing to do

I have rpmforge with priority=10
I have the centos base and update repositories with priority=1

Have I made a stupid mistake with my configuration?

-- 

rgds
Stephen