I found this an interesting work to research. There were several versions of the Guanyin in different Museums, some with their left arm resting on the ground, others where the left arm is held in apose in front of the body. Personally I found the version where the left arm is resting on the ground to be that bit more balanced.
In researching this work I found that the pose held in this sculpture is known as lalitasana (the pose of royal ease) – I think this might be why I prefer the sculpture resting the left arm on the ground – it looks more at ease. I do think that the ‘pose of royal ease’ is a wonderful term though!