[CentOS] yum update success+failure

Just doing "yum update" on two machines (and only two), both centos 
5.10, one of them updated something like 22 packages, the other just 4. 
  Packages which were updated on the first, but didn't get updated on 
the second machine include: gcc, httpd, kernel-2.6.18-398.el5, and 
probably a couple more.  Trying the update on the second machine again 
(a few times), it always ends, "No Packages marked for Update".

So now the first has:

gcc-4.1.2-55.el5
httpd-2.2.3-91.el5.centos
kernel-2.6.18-371.12.1.el5.centos.plus

while the second has:

gcc-4.1.2-54.el5
httpd-2.2.3-87.el5.centos
kernel-2.6.18-371.11.1.el5

WTH???