I've seen the same question on other forums, but have not come across any definitive explanation. The non-cited answers seem to fit into three categories:
- It is due to greater muscle dexterity on one side of the face.
- It is a result of being either left/right brained.
- It is genetic.
Perhaps it has to do with a combination of these three, or none of them at all. Some have even said that with practice they have been able to raise both independently from the other. Why is this so?