Why would it be more than one week out of date ?
If you haven't changed the Auto Update check to manual. Or the system is off-line for more than a week at a time, in which case I would have thought you would do a manual update right away once you connect or avast would check right away when you connect.
Why it isn't showing I don't know, it does on mine if it is even a day out of date if I set it to manual updates. The thing is to do that, it has to check the avast servers for the availability of updates, to do that you have to be on-line.