Not sure where else to ask this, but the "posts awaiting review" figure in the toolbar is always incorrect for me. Often, it says I have three posts awaiting review, but when I go and look, there are none. Alternatively, it says I have 4, but there's only 1 ... most of the time it's out by 3. Any ideas? I know that it indicates it's an estimate (at least, that's what I assume from the ~ symbol), but I find it really odd that it thinks I have something to review when there is nothing.
When we rolled out the "pending reviews" indicator, we had to make an unfortunate concession to the fact that personalizing it for each user is too computationally expensive. The end result is that the number of pending reviews in the top bar is the total count of still active reviews in the system, including ones that are no longer available to you. (Plus some caching may be at play.)
We'd been thinking about ways to make this experience better, but I'm not sure what the status of that is right now. I'll raise it again internally.
The number was made deliberately fuzzy a few months ago. (Despite that post being on MSO, the change applied across the network, prompted in large part by problems on Stack Overflow.) This was done to address a problem that smaller sites like ours don't seem to have, but I couldn't find any discussion of limiting the change to large sites.
So that's the "why", as best as I can tell. (Though I don't know why it's usually offset by 3 in your experience.) If you'd like to request accuracy in labeling on our site, you can make a feature-request either here or on MSE. It'll be seen by SE in either place, but on MSE it might garner support from other small sites, or opposition from large sites. Your call.