Why I can't update packages on Red Hat Enterprise Linux 5 even the package is available to be updated

Issue

Why I can't update packages on Red Hat Enterprise Linux 5 even the package is available to be updated

# yum -v update sysstat
Loading "rhnplugin" plugin
Loading "security" plugin
Config time: 0.078
Looking for repo options for [rhel-x86_64-server-5]
Looking for repo options for [rhel-x86_64-server-vt-5]
Looking for repo options for [rhn-tools-rhel-x86_64-server-5]
Looking for repo options for [rhel-x86_64-server-cluster-5]
Yum Version: 3.2.22
Setting up Package Sacks
Excluding Packages in global exclude list
Finished
Skipping security plugin, no data
pkgsack time: 0.091
rpmdb time: 0.000
Setting up Update Process
Building updates object
up:Obs Init time: 0.663
up:simple updates time: 0.145
up:obs time: 0.002
up:condense time: 0.000
updates time: 2.411
No Packages marked for Update

# yum -v update sysstat-7.0.2-3.el5_5.1
Loading "rhnplugin" plugin
Loading "security" plugin
Config time: 0.079
Looking for repo options for [rhel-x86_64-server-5]
Looking for repo options for [rhel-x86_64-server-vt-5]
Looking for repo options for [rhn-tools-rhel-x86_64-server-5]
Looking for repo options for [rhel-x86_64-server-cluster-5]
Yum Version: 3.2.22
Setting up Package Sacks
Excluding Packages in global exclude list
Finished
Skipping security plugin, no data
pkgsack time: 0.092
rpmdb time: 0.000
Setting up Update Process
Building updates object
up:Obs Init time: 0.657
up:simple updates time: 0.145
up:obs time: 0.002
up:condense time: 0.000
updates time: 2.417
Package(s) sysstat-7.0.2-3.el5_5.1 available, but not installed.
No Packages marked for Update