avast! can check for updates every hour. Antivir and AVG can do that only once per day. Imagine the 24 hour time window without any possibility of getting new urgent update (unless by clicking Update button all day long, but i have other more important things to do than this).
Hmm... Ok. Let's now see what the update holds..
For AntiVir, the threats are quickly injected in database, even if the update is done once a day.
For Avast!... the threats are injected in database between 8 - 15 days after detected (i.e.: just scan files at VirusTotal).
So.. perhaps the updates are done frequently. But they don't get the last protection... because the main Avast! database isn't update yet.
A few examples: Bagle, Virut, Vundo (security toolbar variant), MSN & WL messenger worms (I'm helper in french security forums for a year now, and never seen avast! founing this kind worms).
Sorry for bad english. By the way, I know i'm on Avast! forum.. Just wanna tell the truth, the facts..