Caching. That number is expensive to calculate so it is heavily cached. It will update sooner or later.
It is also not customized per user and shows the total number of reviews in the system, including ones that may be no longer available to you.
I couldn't find where I read it but there are two things that could be going on here:
The counter displays the total number of tasks that are available for review. This may also include such that you cannot do for some reason. Think of questions to close where you already have voted but there are still votes needed to get a close decision or reviews that ...