Duolingo does repetition at the lesson level

One thing I am confused about is that Duolingo does (spaced?) repetition at the lesson level, rather than the sentence or word level. If a language learner is making cards in Anki, they would have vocab cards for words, and also sentence cards, and maybe some other small cards. But in Duolingo, review can only take place by selecting an entire lesson. This means that facts the learner already knows very well is mixed in with the things they are actually trying to learn. It makes scheduling much more inefficient.

It might be a way to combat some of the emotional difficulties of spaced repetition, like Spaced repetition constantly reminds one of inadequacies and Spaced repetition systems remind you when you are beginning to forget something.