Unanswered Questions in the header and also in the forums are actually both just forum queries. Both lead to a list of all the threads that have no responses. Some forums are excluded (such as Off Topic), but otherwise it's from the WHOLE forum, not just the Ask a Question sub forum. Unfortunately the queries are not updated on the fly, they update at a set interval, maybe every 10 minutes, so yes sometimes they show threads that DO have a response.
The Ask a Question sub forum is specifically for questions AND it allows for the posting of questions from non members. This is THE only forum that allows that.