I am sometimes asked to do a quiz. But when I do there are lots of words in it that I have not learned yet! Is something wrong with the quiz or am I doing something wrong?
If you are refering to the quiz that you can do with duolingo plus (and not just the checkpoints of the tree): those test material from the entire tree, so that you can see how you improve from quiz to quiz (though there have beens some reports about the scoring not necessarily being reliable).
I don't find the quizzes helpful at all. I went from a 2.5 on my first one, to three successive 4.9's, and I guessed on many of the questions. In fact, I think the first quiz was harder than the following ones. I think they should make them track with the material that has been covered, but they don't, so I've quit using them.