Sounds and listeners have both a position and direction. ma_sound_get_direction_to_listener() just get's the direction from the position of the listener to the position of the sound and ignores any directional stuff. The pRelativeDirection parameter of ma_spatializer_get_relative_position_and_direction() is calculating the relative direction of the sound based on it's direction and the listener's direction.

So basically, one is calculating the direction between two positions, and the other is calculating a relative direction between two directions. They're calculating different things.