Literal v Idiomatic translation
Wouldn't it be nice if Duolingo accepted more of both? So often I have found the "right" answer to be one or the other without any degree of consistency. Reminds me of the trainee surgeon, given the task of cutting the sutures, asking his boss "Do you want your stitches cut too long or too short today, sir?"
Yeah, I agree, duolingo seems to have problems in being consistent; it is based on crowdsourcing though, so perhaps they believe it'll fix itself in the long run with the users' suggestions. The only problem with that is that of course the earlier lessons are much more tested (if a bit silly at times), and the later ones end up being a lot of trouble for the users.
The basic rule I think it follows is that if the literal translation makes sense and captures the meaning (or one of them) in the target language, they go for it, otherwise the translation is idiomatic, sometimes ignoring a lot of other "midway" translations that still make sense.