The program updates are released but to avoid excessive load on the servers the auto update check happens randomically every 7 days and it's not checking every time you connect to the Internet as with the iAVS checks. Every avast installation has a unique, randomly generated GUID ("general unique identifier") associated with it (it is generated during installation). This ID is random, but fixed. They use this value to determine when the updater pops up on YOUR machine, i.e., the program already knows that the update is there, but it won't tell you until it's your turn. You have to allow the update, i.e., it's not a forced and automatic update.
Please, post the last 300-350 lines of avast log: C:\Program Files\Alwil Software\Avast4\DATA\log\Setup.log