I believe all the downloaded packages are tested - and if they are found incomplete/broken, they are not installed (installing a broken package would make the installation unworkable, not just “missing some samples”).

As for the downloaded files - if you tried to download the files very soon after the update was released, it’s possible that the web wasn’t updated yet at the moment (the incremental update is delivered faster, of course, because it’s smaller).
What version of the virus database does avast! show now? What happens when you invoke the iAVS update from avast?