[CentOS] yum --security not detecting security updates

Hello,

We are running CentOS 5.5 on a server that is not reporting any
security updates:
[root at server01 ~]# yum -y --security check-update
Loaded plugins: fastestmirror, security
Loading mirror speeds from cached hostfile
 * base: bay.uchicago.edu
 * extras: bay.uchicago.edu
 * updates: mirror.nyi.net
Limiting package lists to security relevant ones
No packages needed, for security, 261 available

However, Nexpose, our vulnerability scanner detected otherwise. Upon
digging deeper, I noticed that we are on a kernel version that has a
known issue fixed in a later version:

[root at server01 ~]# rpm -q kernel
kernel-2.6.18-194.el5
kernel-2.6.18-194.8.1.el5

http://rhn.redhat.com/errata/RHSA-2010-0610.html
http://lists.centos.org/pipermail/centos-announce/2010-August/016890.html

I appreciate anyone's insight in helping me understand this a bit better.

Thanks!