Practice session is actually lesson
I just had an entire practice session based around "worüber sprechen". Except I'd never seen that in any of the lessons; "worüber" was a new word for me (it wasn't flagged as new, however). So this practice session was actually a new lesson. Is that something Duolingo does often? Did the German lesson that was supposed to introduce worüber simply miss it somehow?
All the words taught in a skill are listed in the skill page. If you notice, not all words are tested when one completes the lesson the first time. Redoing a few times help bring up more new words. But sometimes, a few words are not picked even after a few redos and may popup during a practice session.