Because duolingo is full of mistakes. Just as bad is it that in this course, nearly half of all sentences have some words that are pronounced wrong, sometimes even changing the meaning of the word - because they've chosen to use genetic computer generated voice input, rather than have real humans record the sound of the language, or at least correct the voice input that is wrong.
Regards, a Dane reviewing duolingo in my own language, before accidentally wasting a lot of time trying to learn a language for real with this. If this software teaches you things that are wrong again and again in one language, I don't believe that it's going to be different in another. Too bad for the people who wasted their time and effort learning words and sentences that just aren't correct.