@Xai:
I wonder. Is it possible that they are not "predicting" Luffy's moves, but just "seeing" it too fast? Sort of like "spider sense" if you will.
the only problem with that is..
http://www.onemanga.com/One_Piece/519/10/
She couldn't have saw it. With her eyes being closed and all.
Also to the threads creator. As of right now, looks like you hit the nail on the head with your theory.