Nothing is perfect in life, more so in the English language, nevertheless I shall attempt to defend "unanswered" and argue that the term is appropriate, it has its flaws but it is the best solution for Stack Exchange's purposes.
If I click on the sandwich menu and visit the page unanswered questions, here on Meta, the vast majority of posts do not have any "answers". There is also a preface at the top of the page which says
11,385 questions with no upvoted or accepted answers
The explanation is written in good concise English, clear and unequivocal. The paradoxical situation the OP speaks of “ I was very puzzled as to why there were answered questions in the tab named 'Unanswered'” should have been dispelled by that brief clarification.
In stark contrast, if we click on Questions and then on the unanswered tab, no clarifying message is supplied.
Objectively this is counter-intuitive but there is a simple fix. The same message should be displayed on both pages so as to eliminate, as far as possible, ambiguity and confusion.
Let's now talk about semantics, hopefully I'll be able to illustrate why "unresolved" opens its own separate can of worms.
- Can a Feature Request be resolved?
According to the Longman's dictionary, problems, disputes, conflicts, issues, situations and matters are resolved or stay unresolved whereas problems, cases, riddles, and mysteries are either solved or unsolved. On the other hand, requests are responded to, approved, accepted, granted or they can be refused, rejected, turned down and ignored. In other words, a request is not answered.
In fact, on the first page there is a well-supported feature request that has received four "answers", none of which have a positive score. I don't have 10K rep, so it's even possible that there are also deleted "answers". Technically, SE could label those pages unapproved or ungranted, but then we would have users pointing out that topics of discussion on Meta cannot be approved or granted. Discussions can lead to solutions but many also end up unresolved despite receiving three or more upvoted posts, basically people just going round and round in circles and rarely meeting in the middle with little or no agreement reached.
Questions that may not necessarily have a clear-cut right or wrong answer and are often subjective. If your question isn't a bug report, feature request, or request for assistance, or question with a concrete answer, it's probably a discussion.
How many more questions from the 88,979 questions posted until today would MSE have to add to that unanswered previously mentioned?
Leaving aside Meta, let's move onto the 174 sites that currently populate the Stack Exchange network.
The slogan that greets first time visitors is the following [emphasis in bold mine]
Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Then we have the following tagline [emphasis in bold not mine]
The right answer. Right on top.
Experts like you can vote on posts, so the most helpful answers [emphasis mine] are easy to find.
The message is all about one question with top answers. The noun "answer" is used in the manifesto, not resolution, solution, result, or elucidation. Therefore, it makes perfect marketing sense to use the terms answered and unanswered repeatedly and throughout the entire network. Simplicity is always best
Last but not least, an answer is not only a post submitted by an ‘expert’ but it can also offer the solution to a problem posed by the questioner.