This is a known "issue", but it's definitely by-design. The number shows the total amount of items still in the review queues, including the ones you already handled yourself but which might still need additional reviews to get removed from the queue.
This, as well as the reasoning for it, is explained in the answers to this related question, which also explains why you noticed that in the last days. It is actually rather common for there to usually be some small reserve of pending reviews inside all the queues1 but you only noticed that lately because this number is only shown for 10k users with access to the moderator tools, a privilege you only gained recently.
1) Well, in a perfect world the counter should be emptied as fast as possible, but the users of this site can only do so much in reviewing and as long as the number doesn't grow large over time and there's a continuous fluctuation, it's not something to be particularly worried about.