Why are some dead bodies in movies depicted as floating when others sink?

Because people in hollywood don't know exactly how things of that matter work. Some directors might have put in the work to find out the truth for themselves. Other directors just assume that what they know is correct. The real answer is that the lungs hold a bit of air until displaced by water. Objects in water stop sinking when they displace a mass of water equal to their body mass. But take into account the gases that our body produces/Methane created by bacteria in the body, specifically the stomach area. Then the body will start float to the surface like a cork. There are many factors to such a question. It all depends.