Unfortunately, there isn't an industry standard on what truly is an "infected" file. Symantec classifies files as infected if they contain viral code. Others don't.
As an example, say a worm infects a machine, changes the desktop background to a picture of a kitten, then spreads across the network. Symantec would look at the .jpg that's dropped as part of the payload, see that there's no viral code in the file and leave it alone. Other vendors may flag it as infected and remove it, even though it didn't contain any viral code...it's just a picture of a kitten.
The best bet would be to submit the file(s) in question to us so we can get a look at them. It's possible that this is a new variant that we don't have definitions for yet...we simply won't know until we get a sample and have reviewed it.