Why are some apps hidden and how to unhide them
Android has many apps that run either in background (such as Accuweather daemon) or only appear as widgets (for example TypoClock). Since these two apps don't need the user to open them from the launcher, they don't add an icon to it.

The same happens with Live Wallpapers. If you use a root explorer or another root app that will let you see all the installed apps, you will see that the list is higher than the apps that appear in the launcher.