They could hardcode detection for that set to identify the "test virus" that many "researchers" use. It appears this gives AVG less than idyllic ratings. I see both sides on this one.
(researcher) "It didn't detect the test virus."
(pundit) "That's not a real virus."
Two ratings on AVG AV:
PCWorld - Grisoft AVG Free Edition 7.1
This antivirus program lacks key features and adequate heuristics. None of the three free antivirus products tested for the "The New Virus Fighters" ranked highly, but Grisoft's AVG Antivirus Free Edition holds the distinct honor of having placed last among all ten paid and free contenders due to a clunky interface, the omission of significant features, and below-average performance.
CNet - Grisoft AVG Free Edition 7.1
This antiviral program offers rock-solid protection for no financial commitment. AVG Anti-Virus Free Edition provides all the necessities, including a tool for scanning your hard drive and e-mail, as well as a real-time shield to prevent infections.
Again, what's wrong here? Is CNet on the Grisoft payroll? Is PCWorld full of idiots? Or the reverse?