It is caused by brain asymmetry.
The left side is a facial expression that received a signal from the right brain.
The right side is a facial expression that received signals from the left brain.
The right brain is creative, emotional, perceptual, emotional, and intuitive.
The left brain is thinking, logic, analysis, reason, rational.
The left side of the face is likely to show the true expression, and the right side is likely to show the desired expression to the opponent.