I've noticed that sometimes I get prompts that say they are for new words, when I'm sure I've studied the word before. This just happened with 倾销, and when I click on the magnifying glass it gives me stats indicating that the words has indeed been studied before (last studied two weeks ago, 50% success rate, etc.), but on the study page it still says new word. I think maybe this only happens on the website with words that I've previously studied on the app, but that's just a theory.