The vertical pose rotation of armor stand arms pivot from the side of the arm, rather than its center.
Why is this an issue?
It overcomplicates map makers getting precise poses/positions/alignments with armor stands for visual use by offsetting the arm's pivot point.
Steps to reproduce:
Run commands:
In chat:
/scoreboard objectives add pose dummy
/summon armor_stand ~ ~ ~ {ShowArms:1b,Pose: {RightArm:[0f,0f,0f]},Tags:["pose"]}
Repeating:
/scoreboard players add pose pose 1
/execute store result entity @e[tag=pose,limit=1] Pose.RightArm[1] float 5 run scoreboard players get pose pose
Notice the arm rotates from the inner side of the arm's top, not its center.
Oh, it absolutely doesn't prevent map makers from doing that; the model can easily be centered on the side.
That said, the Pose.XXXArm rotates the shoulder, not the lower arm. If you turn your shoulder, it rotates from the side adjacent to the body, not the center. All values rotate from the shoulder, not the lower arm, so this seems intended to me. (You move the arm by changing the joint, which is the shoulder.)
It's also worth noting that changing this may mess up existing maps relying on this.