Perhaps because these sorts of phrase aren't literal translations, so Duolingo needs to give you a little more leeway. E.g. Would you differentiate between "See you" and "See you later"? The more literal difference between また and またね would be "Again" and "Again, ay."
the ね is just a nice filler, it does not carry any meaning as such. Someone said it reminded him of "eh" in English and that's what I stick with.
It is the gentler form of よ at the end of a positive sentence.