Review-needed indicator is trigger happy [duplicate]

Question

Since I recently gained 2K, and quickly thereafter 3k reputation here on MSE the review queues became accessible to me. Every now and then I check on them to see if there is something that needs reviewing.

When I notice a red icon I'm under the impression that there are a lot of reviews to be done, but every time I get disappointed when after 1 review the queue is empty again. So I decided to look for the logic behind the different states of the review-needed indicator, which led me to this announcement by Jon Ericson. In which there is stated that the default values for the red button are:

Queue                 Default
-----          --------------  
Close                      20
Reopen                      5
Low Quality                 4
Suggested Edits             3
....                      ...

Apparently this isn't the case, so what exactly causes the review-needed indicator to turn red the instant there is a review item?

Secondly, is it desirable that there is always a red flag, instead of the gray flag when there simply is something to review.

Regarding the suggestion that a filter might limit the amount of review tasks available:

In that case there is a message displayed:

There are no items for you to review, matching the filter "[discussion]"