So far, I thought that the number shown in the orange box in the top-bar was the number of items available for review. However, when I acually visit
/review, the numbers don't match:
Note that top-bar shows 15 items (?) but the queues only add up to 8.
Since I can't think of a reason for this being by design (which, as experience shows, is not a good indicator for "this is not an SE feature) I'm tentatively labelling this a (UI) bug.