it has to do with Martial Artists vs the regular people/civilians.
Civilians (scholars) have the left as the fist and the right as the palm. The idea was that most people were right handed so to open up the hand into a palm would be a gesure of peaceful gathering. Kind of like in the west as shake hands (right hand - prevents us from pulling out a knife, most of the time
)
Martial Artists are military and religous organizations.
The weapon is held predominantly in the right hand so the palm is used to cover the weapons hand (kind of hard to change hands).
later