The red dot icon has always been a bit buggy in my opinion but this seems to be status-bydesign from what I can find about how the indicator works. (It might have been updated but if so I missed it). According to this main meta post the red dot will show for a queue if there are >=2 items in that queue at the time. That post also mentions the following caveats where number 2 is probably why this is occurring here.
People are no longer notified of review queue tasks that they can't access because of their privilege level and
People might still be notified of review tasks they have already completed if they don't get cleared after an hour
The main red dot will then show if:
if any review queue has a red dot (explained below) and
if it’s been at least an hour since you clicked on the review icon.
Though note the figures in this post for individual queues N were adjusted per the first meta I linked.
So that is why you're seeing it, it is just a false positive because you've already completed the review an hour or so ago but the review itself has yet to complete.
For the differences on the grey and red dot then are the grey dot shows up if there are any reviews in the queue but lower than N and the red dot shows if the amount is higher than N.
The red dot probably lags behind and shows when it shouldn't because of the above caveats and shouldiblamecaching.com.