I accidentally noted that a particular question does not appear on the list of questions when I am not signed in. The question comes up in the top five once I am logged in. Why is this so?
Is it a bug in the feature?
As Charles notes, the list of questions on the homepage is altered for anonymous users: instead of listing recently-modified posts, it attempts to pick questions that may provide a good example of what the site's about.
Currently, that means it's very similar to the monthly hot questions list.
If you want to view a list of recently-modified questions without logging in, you can find it in the active questions list.