Downloading new songs added to a playlist

[KayLay]

I play my Spotify music by going to the playlist of all my favourite/chosen songs. I'm guessing it's a playlist on its own created by me clicking on the plus sign of a song to add it to my songs? The other day I signed up for premium and then downloaded all the songs in the playlist. But, as I go along listening to new songs I add them to my favourites and they get added to that playlist. Today I tried to download just the new songs so that I can listen to them offline and it has started downloading every single song on the playlist all over again. What's more, I turned my wifi off to see if my old songs from the playlist were still downloaded to play offline and they weren't. It seems such a simple concept that you should just be able to download songs that haven't yet been downloaded in a playlist. It's just got me confused.