Because the adjectives also take an ending many times. In this case, it is the accusative case without an article anderen (m), andere (f) anderes (n) and andere (pl).
This got me too! I got away with it because Duolingo thought it was a spelling mistake (phew) but I didn't get it. Thanks kyky - I didn't realise that there is no acceptable shortened way in German like there is in English (where the thing that is possessed is implicit). Sounds like a potential Blackadder sketch: Is that your horse? No, that is someone else's horse... usw.
Can "anderes" be used in all of the same type of situations as the English word "else?" By that I mean does "anderes" directly translate to "else," or is it just similar and only applicable in some of the same situations, and would it be applicable to some situations in German that would not be fitting for "else" in English? Was that word soup? I tried to be quite clear.