It sounds like maybe you are looking at the "Applications" sections a few entries above "Installations" and that does show everything, including apps installed as part of the OS. But the Installations section just shows apps that have been updated since the initial OS install. If you look in the Source column you can see which ones are from Apple.
For example, in mine I see a Security Update 2013-003 installed from Apple on July 2, 2013.