You are right, only one file is quarantined. However, the rest
are not on the loose. It means that the files have either been
deleted or healed.
Quote from AVG Help Topics:
The Virus Vault is used to lock up infected files out of harms
way. You can use the Virus Vault to heal "vaulted" files and
restore them to their original location on your disk. Files are
moved to the Virus Vault during tests or by resident components
such as the AVG E-mail Scanner.