Why Have Many Experiments Somewhat Underestimated People's Ability to Recognize

Why have many experiments somewhat underestimated people's ability to recognize facial expressions of emotion?​

A) They used photographs in which all faces were posed looking in different directions.
B) ​They used photographs in which all faces were posed looking straight at the camera.
C) ​They gave people a list of emotions and asked them to match faces to the list.
D) ​They asked for fine distinctions, such as the difference between guilt and shame.