I just saw pirates of the carribean 4 and had a question about something i saw in it about mermaids?

half way through the movie serena was in a glass box filled with water. the cage broke. all of a sudden she had legs. you could see the knees and such right before she grew the legs through the fins. then towards the end of the movie she had fins again. how does that happen? how can a mermaid have fins one second then legs the next and then back to having fins again?