Thank you Lanny for the quick reply!
It is iWork 09 and the same is true for iLife 11.
Actually, all the apps are up to date, since the update process from within the apps still works fine. Luckyly, I never had the problem that some others here had, not being able to update.
So my issue is quite minor. It just bothers me that there are 8 apps in my list of purchased apps with the option "install", while they are actually installed and up to date.
I found that when I click on install, MAS downloads and installs the app again (not a double install) and then everything is fine. So if this is the only solution, it just takes quite some time to download the apps again. I was hoping that there is a way to make MAS recognize software that was not installed via MAS download.