I imagine that if it's in Quarantine, it's reserved by the Antivirus, after you pay the corresponding update, you'll get the dose needed to kill the virus, recover it and take it out of Quarantine.

The deal of the century.

Because it is most likely deleted and only the notification remains, or because AVG needs to identify you as an administrator user to obey. I've long since stopped using AVG for being resource-aggressive and a bit disobedient, and I've adopted Avast.