What does the number next to the review link in the top bar actually mean?

Question

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".

Answer 1

The numbers on /review are a lie (and caching) and is discussed before on MSE

The topbar counter jumps when you reach 10K probably because it unlocks the moderator tools privilige which will give you access to the full review history of all users in all queues.

You can reconstruct the number you see in the topbar if you take a look at this SEDE query (updated last saturday).

count               |name                |name 
-------------------------------------------------               
8221                |Close Votes         |Active              
   2                |First Post          |Active              
   5                |Helper              |Active              
 162                |Low Quality Posts   |Active              
1412                |Suggested Edit      |Active              
  19                |Triage              |Active              

Notice that the close vote review count was exluded from that number of available reviews in the topbar. If you leave those out of the above numbers we get close to the 1600 you see.

As an alternaive to those number from SEDE you can visit the stats page of a review queue but make sure you logout or open that in an incognito window. By doing so the count will reflect the actual number of reviews because your skipped/flagged items are no longer substracted. (thanks to @Woodface for this tip)

The suggested edit count is most off in the /review page and that has probably to do with the review-lock that is applied to suggested-edit reviews as explained in this answer from Geoff Dalgas

It's a bit like ticketmaster, or any type of reservation system. When you visit a suggested edit review task it's now "checked out" to you for that time - the counter previously didn't reflect the amount of "checked out" tasks.

So to conclude:
- In the topbar you see all available reviews site wide (excluding close votes on SO)
- on /review you see the number of tasks available for you personally.

Depending on locks, caching and an occasional review ban those numbers vary.

^{This post copied from my answer on MSO and adapted for this question}

Answer 2

It counts every review item that’s available, even if they’re not available to you.

Quoting @Shog9 from an answer on Meta.Physics:

This number counts everything – including tasks you aren't personally eligible to review. This includes tasks you've already done, tasks you've skipped, and tasks that the system expects you might not be sufficiently impartial to review (e.g., things you've already flagged or posted).

The idea is that even if you can’t tackle all of those items individually, if you notice the number spiralling out of control, you can encourage other people to give the review queue some love.

That’s why you can’t juggle the numbers on your /review page to fit – there must be some review items that are being hidden from you.

A few caveats:

• The formula is slightly different on Stack Overflow, owing to the somewhat monstrous size of the Close Vote queue. Quoting @AnnaLear:

  We are excluding the Close Vote queue from this on Stack Overflow, and there's a minimum of 10 pending reviews that have to be in the system before the indicator will show up.

  I haven’t got dev confirmation, but I think the minimum-of-10 reviews filter only applies on Stack Overflow. I’ve definitely seen it at less than that on smaller sites (and if not, I’ve obviously misunderstood this statement).

• As with all review counts, it is subject to caching and thus may be a little inaccurate at times.