Q: Preinstalled iWork Apps

You must download the updates from support.apple.com/downloads/. Because you did not purchase them on the App Store you cannot upgrade them on the App Store.

First of all, have you checked if they are on the Applications folder? Open Finder, select Applications on the sidebar and check it. If not, put them on Applications.

If they are already on Applications, reindex Spotlight on your iMac > http://support.apple.com/kb/TS4236

Now, I think that those apps are registered to your Apple ID, so they appear as installed from the App Store. Have you tried the steps of my link in order to know if your problem is solved?

Because there are multiple but different versions of the same software. There are App Store versions where each application is sold separately so they can be installed separately without separate support files. Then there are DVD versions and Pre-bundled versions. These versions install the same basic applictions all at once coupled with separate support files that all three use.

Because these are different you cannot update them with the same update software.

Simply re-installing the OS will leave your apps intact. Erasing and re-installing won't.