Or maybe they dump their energy very efficiently. A SWC with a wide meplat or a jacketed hollowpoint has alot of frontal area and is going to stop short in sand.

Try this analogy- you are sitting on a beach, first try jamming your hand in the sand fingertips first, then try jamming your hand in the sand with a balled fist- which do you think will penetrate further?

Big slow bigbore bullets are just as effective as high velocity small bullets, but for entirely different reasons.