If you change any of the Update Parameters (the area I mentioned as I think that can stuff up auto updates) e.g. My computer is permanently connected to the internet. As doing the same for dial-up if you were on it some stuffs up the auto update.
My best guess on the "The update was unsuccessful" meaning, is that the update attempt was unsuccessful, just what it says not that there was or wasn't an update available as we don't know at what point the update failed.
What I do know is what it doesn't mean, if there is an update check and no update is present then it completes with success and reports "Last update attempt (plus the date time group), rather than "The update was unsuccessful."
Again, is means what it says on the tin, you asked for the last pop-up message and that is what you got, the last pop-up. A successful update check if there was no update available doesn't display a pop-up, so what you are seeing is the historic, last pop-up displayed.
For me the last pop-up was a success and an update was done (see image), so I got the update "Your virus definitions have been automatically updated," even though the last update check succeeded but there was no update available.