It seems to me that when I select "Not Sure" when reviewing an item that the item gets removed from the review list (The number in the review section seems to decrease much faster than the number of reviews each day). Is this just a coincidence or does this happen? Or is this the wanted feature?
It gets removed from your review list. If you aren't sure about this, you can easily experiment by picking a small category and hitting "Not Sure" 5 or 6 times in a row and looking at the count go down. And, the reasoning makes perfect sense. If you are not sure what to do about it, there's no reason for it to stay there. But, if you are not sure what to do about it, then someone else should look at it, so it is going to stay in the queue for others to look at. It is simply removed from your queue.