The review queue descriptions are a bit brief, which leads to a lot of confusion. The "items that need review" leave off from the description "that you can perform". The number doesn't reflect the total contents of the queue, just the number available for you to review. Some examples of what is excluded from the count you see:
- Your own posts or edits (you can't review your own stuff).
- Things you have already reviewed that still require additional reviews by others.
- Things you don't yet have sufficient rep to review (users can participate in different review queues with different rep levels).