Subforums with numbers in parentheses?

I've noticed that some of the subforum names here include numbers in parentheses, e.g. "Addons (4)" and "Nuclear Engineering (1)". Does anybody know what those numbers signify? They don't seem to correlate with thread count, post count, number of further subforums, or even stickied-thread count.

Oh, ok. I hadn't thought of that possibility. Thanks for letting me know.