The icon that displays
X total posts awaiting review will often show a number (10 in the screenshot). Then when I visit the review page, there are no items waiting there. What gives?
I believe it may also be a function of what may be in the queue that still needs to be resolved. I frequently see the queue remain at a certain number until all of the posts that still require votes on them to reach resolution have been cleared.
This seems to occur in the Close Votes Review Queue where posts remain until they've either been closed or have received enough + and - votes to no longer be in contention of being closed. It's at that point they seem to finally get cleared from the count of what's open in the queue.