A couple of days I noticed that when I click the pending reviews notification I go to the review list but the number is not the same:
As you can see the notification is showing 3 but when click there is nothing do to.
I'm missing something?
From what I've read on the subject, it seems that for moderators (and users with the moderator tools privilege) that number shows the total amount of open review tasks in the system, not just those available for you. The purpose of that number, then, is to help you keep an eye on whether or not the community is managing to stay on top of the review queues or not. If the number keeps getting bigger and bigger, then either the users with review privileges aren't actually reviewing posts, or there simply aren't enough such users on the site (not likely in our case).
I don't know all the possible reasons why a review task wouldn't be accessible to you, but the most probable one IMHO is that you've chosen to skip some of them. When you skip a review task, you can't go back and complete it, but it will still remain available for other users, and as such is reflected in the number you see in the toolbar.
For more information, see this post at Meta Stack Exchange.