SEs do not update their whole database at once. I do SEO for many different clients with a wide range of products and services. I've noticed that just because some engine updated their listings for a particular category doesn't mean I should expect listings to change in another category. I dont mean directories like Yahoo I just couldn't think of a better word for 'category', topic maybe.
When a SE says they updated every 3 weeks it doesn't mean they do nothing for 20 days and then update the whole thing. It means that from any given point it will take 3 weeks to get back to that point.