I've noticed this as well. In trying to understand what is actually going on in these situations I've found the answer to be quite complex. Apple's information on this subject is somewhat nebulous as well. Here's a snippet from Apple's System Overview doc's related to the subject.
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The way the Finder in Mac OS X builds its databases is also different from the Finder in Mac OS 9.
¥ The Finder first adds applications at boot time by scanning the standard locations for applications in the user, local (plus system), and network domains.
¥ When users navigate through the file system, the Finder adds applications in each visited directory to its databases.
¥ When users try to open a document or attempt any other action that requires an application, and the Finder cannot find an appropriate application, it displays a dialog, allowing the user to select an application. This application is added to the user's application database.
Because there may be locations in the file system a user has never visited, or documents of a type she has never attempted to open, the Finder might have an incomplete view of the applications available on a system. Yet it has a built-in capability for "lazily" updating its view of the file system."
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The last sentence is what I was referring to as being nebulous. "Lazily" doesn't seem to be defined in subsequent pages. There is also mention in the "Inside Mac OS: Performance" pdf that discusses tracking file system changes. In particular the Carbon event manager (since our focus is on the Finder, a carbon app). Apple seems to be pushing developers to not use a constant event polling model till a window becomes active (via the kWindowActivateEvent event handler) to reduce CPU cycles an app uses. So it might be safe to say that this is stirred into the soup as well if Apple follows it's own advice. As you can see it get's kind of hairy involving more than just a simple refresh interval in an event loop. There are other forces at play here which require an in depth understanding of how the file system manager, finder, event manager, and who knows what else, work together.
Pretty interesting I think. Perhaps one of the more experienced boards members can shed more light on this.