In this lesson you just can’t mark all the words because they are hidden. If I finish the lesson, those 25 rubbish-words would go to my known-words list.
Screenshot of the lesson with all the available words lingQued:
In this lesson, as many as 68 words are hidden: Login - LingQ
I mean that as you have gone through the lesson and have marked all the words, you expect the counter of the remaining words to be zero. As you can see on the screenshot - it is not.
Actually I looked into edit version, it looks ok. This remaining number seems to be a bug, which if guessing, is caused by quotation mark symbols or such.