What is the actual source of the number displayed next to the review link in the top bar? I found Notification for reviews shows way too many (via Review flag count is still wrong and similar) but, and apologies if I'm being dense, the answer there didn't seem to have an explanation of where the specific number comes from.
I can't make any combination of task counts on /review (that are visible to me, at least) add up to the number in the top bar.
I had thought that perhaps it was a count that just lagged a little behind the /review counts, but it changes often and quickly enough (and is so far off from the other numbers that I see on /review) that I can't imagine how this could be the case. Perhaps there are items or queues that I do not have the privileges to see, but I don't know what those are, and finding out what they are (if that's the case) is part of this question.
I've also been told in chat that it is actually the "number of users who have black hair and ate a croissant in the past hour".