This bug maybe related to the poll (notification push), when staffs deal with a review, it will push a notification containing the number of pending reviews remain to all the moderators.
However, category moderators can’t see all pending reviews, so the number of reviews will suddenly become extremely big(contains reviews of all the categories). And when they fetch /review/user-menu-list, the number will be correct again.
Not so sure about that, give a bug report for investigation.