Searching for nonexistent packages

Hello,
today I tried "yum -y update" on some Fc11 machines. 
During the download I get the messages, that several packages, like 
dhclient-4.1.0-22.fc11.i586.rpm
cannot be found on any mirror.
It's not a big problem here, I guess the updates are quite unimportant, but I 
am wondering, why yum wants to download these packages. There seem to be some 
inconsistencies in the Fedora file hierarchies. 
I already called "yum clean all", because I thought that maybe the local cache 
is in some strange state, but that didn't help at all.

Best wishes
Alois