from what i experienced so far, it is quite common. For once, words learned in new lesson have an impact on older lessons. Besides that, the strengthening lessons often use different questions then what the lessons has thrown at you before. I started to see the lessons as vacabulary learning and the strengthening as a way to test it with different question. however, every then and when new words are introduced that are not marked as new aswell. It can aswell to happen to have multiple new words in question - non of them marked as new. It is verry confusing, however, i do not think that is all so bad, as the way you learn from these sentances does not verry from the way you learn it elsewise.