"See you soon."
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 asking for confirmation, so like "we'll see each other soon, won't we", but very informal and colloquial speech.
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.
I don't know why, but i learned じゃまた as "See you later" from another course. Maybe また is a short form?
Yeah i reckon it's shorter as well because, じゃ should make it "well, see you soon".