I'm going to give it to Samwise. What happened was when they filmed the scene, the horses were galloping, but the hoofbeats weren't picked up by the microphone. So they went back and added the sound in, but they accidentally added in the sound of cantering horses. (A canter is a bit slower than gallop, if I'm not mistaken. Plus, a canter is three beats and a gallop is four beats.) You can read about it
here.
The thread's yours!