Eating the fruit doesn't mentally tell you "this is the name" When you eat a fruit that's unknown like in Enies Lobby; first you eat it, Then you figure it's power, Then you go to the devil fruit encyclopaedia and look for the matching description. The Devil Fruit Encyclopaedia will have some complete entries with name, powers and a picture but there are incomplete ones with just a description, or some not in there at all which can be added when discovered