youtube playlist

why does this playlist doesn't work?

http://www.youtube.com/playlist?list=PLD1F9B84726BCA762 - works
http://gdata.youtube.com/feeds/api/play ... 4726BCA762 - doesn't work

and this works

http://www.youtube.com/playlist?list=84C10A9CB1D13841 - works
http://gdata.youtube.com/feeds/api/play ... 9CB1D13841 - works

Remove "PL":
http://gdata.youtube.com/feeds/api/playlists/PLD1F9B84726BCA762 - doesn't work
http://gdata.youtube.com/feeds/api/playlists/D1F9B84726BCA762 - works