I have no idea what type of detection that is - but it doesn’t really matter, you are wrong in your assumptions.
All the detections are more or less generic (the only exception would be a full-file hash, but avast! doesn’t use anything like that for detections). Even the most “strict” signatures may detect more in avast! 5 - e.g. because avast! 5 has better unpackers, and is able to extract (and subsequently scan) embedded objects from the scanned file. Or, with the help of the emulator, it decrypts an encrypted file, even if unsupported by the unpackers. Or, different parts of the file are scanned (than in avast! 4) because the engine is able to change the behavior in a more clever way.

So, saying that “avast! detects the file, thus the virus lab must have seen it” - is basically never true.

As for mentioning why avast! 5 detects the sample - I’m afraid I couldn’t, because I simply don’t know (since I don’t have that sample, for example).
But yes, I think it’s quite likely that the reason is simply the fact that the virus database on VT is obsolete for avast 4.