yum upgrade on 3.4 and spamassassin

[frcaen at gmail.com (Francois Caen)]

Hello,

I run CentOS 3.4 on 3 machines (i386). Original yum.conf

If I do a yum update: 
No actions to take

If I do a yum upgrade a machine that has no spamassassin: 
No actions to take

If I do a yum upgrade on 2 machines that have
perl-Mail-SpamAssassin-3.0.1-1 installed (3rd party rpm, more recent
than the CentOS/RHEL one):

I will do the following:
[update: spamassassin 2.55-3.4.i386]
I will install/upgrade these to satisfy the dependencies:
[deps: perl-Time-HiRes 1.38-3.i386]
Is this ok [y/N]:

Could someone please explain this behavior? Why is yum upgrade trying
to upgrade a package that is a newer 3rd party rpm, with a different
name?

Thanks,
Francois