Shoulders
A body in its playing position usually is showing one shoulder a bit higher than the other one. The shoulder of the resting arm on the table is usually lower. This is being enabled because of the twist of the body while forming the stance.
Should both shoulders be equally high, the elbow of the cueing arm usually is not straight on the cue but pointing outwards. With the classic stance the cue will practically always go underneath the body while cueing, and as a result the tip of the cue will go upwards. It's just physics.