How did the AVG8 virus get past Avast!?

One of my computers became infected with the AVG 8 virus. It was very difficult to get rid of. Avast has done a good job in the past of removing/isolating viruses. How did it miss this one?

Fact of life, no security program have 100% detection…

Fake antivirus overwhelming scanners
http://www.networkworld.com/news/2009/100209-fake-antivirus-overwhelming.html

Remove Antivirus8 or Antivirus 8 (Uninstall Guide)
http://www.bleepingcomputer.com/virus-removal/remove-antivirus8