Apple's software update doesn't find any updates?

Since updating to OS 10.8.3 the software update does not seem to work anymore. Recently iTunes came up with a message saying that a new version was available but clicking on the update option it re-directed to the App Store and a notification appeared stating that no updates were available. Doesn't make sense to me, especially if I search online on the Apple website I find plenty updates!

Is this a known glitch or am I doing something wrong here? This problem has occured on both my iMac and MacBook Pro. Bye the way, an update to 10.8.4 hasn't changed a thing.