Well if you start with a forum search for av-comparitives you will find a few topics on this very thing.
The last test (retrospective) effectively is looking at proactive protection, so those AVs with heuristics can do better in this type of test. avast doesn't use heuristics in the on-demand scan but has started using more generic signatures to detect variants of a type of malware so that should improve detections over the 4.7 version I believe.
