It has something to do with the countries that got this...The English version of the google pack doesn't get avast!, but others do...like the French version...
That said, avast! should be known to google...
Actually, looking at this, it would recommend avast! if you chose the pack and were in say France, but we aren't so avast! isn't there...
And they can't list avast! in the list either because the explicitly say that they have no connection with the companies the list...