Translation:The children are running not on the sidewalk, but on the lawn.
It gives one translation as the accepted one, but also accepts other alternatives, but it isn't going to mention them all, or you'd have about a page of writing and we'd all be even more confused. I'd say pavement, and it accepted that, but it's got to be usable for people who speak various kinds of English. I like it to accept my British English, of course, but that's not really going to be the number one around the world, is it? Much trickier is when the grammar is different- so in US English, you'll often not use a preposition which it would be strange not to use in British English. I don't know where Duo is available, but if you factor in Australia, NZ, India, various African varieties of English, to name but a few, then you have probably many many requests to accept variations on each question... A lot of the users here are learning using English as a middle language, so it has to be accessible to them, too. If you think of the number of ways you could translate this into English... the makers of the course have a BIG job to do to keep us all happy!
Here, here! snosage - very well said. A lingot for you! If we as students are flexible enough to try to learn a foreign language, then surely we can utilize that same flexibility within our own native language and just accept there are different ways to express something in English, depending upon in which school, city, state, country or even continent you studied it.