They can't put in every possible translation and regional variant anyone might use. There's always going to be something forgotten. If you think it should be an option well then, they have provided the 'report' function for you to suggest the alternate translation. It might even get added.
They are synonymous. In Italian, the number one will change gender; just number one, mind you. Here, because of salsiccia, it changes to the feminine 'una.' In English, we usually say 'a sausage' instead 'one,' but they are both correct. If I were to say, "I am cooking a sausage," how many would you say I'm making? That's right, one. =)