Earlier this week the C++ tag showed about 13k followers. Yesterday it showed 19k. Now it shows 14.8k.
Why this variability?
13k is the correct value for C++ at this moment - there was a bug in the way cached values were being pulled for display.
Off-hand I think it has to do with using "short-hand notation" and the way the calculation is done. I subscribed to a few tags, then later on (After being plastered with emails) unsubscribed. When I did so (after a fresh page refresh) it would say "15k subscribers", then I would unsubscribe and it was like 14k. (Though the site is popular, I can't image there are that many active users, and more specifically those targeting that individual tag).
I almost feel it doesn't re-display the count, it just takes the value embedded in the page and subtracts/adds (regardless of which notation is being displayed).