rest position:
head (0,0,0)
tail (0,0,1)
roll 0

pose:
rotation_euler (0,0,-pi/2)   # With respect to rest position, this is actually (0,pi/2,0)
position (0,0,10) # With respect to rest position, is actually (0,10,0)

# Same bone with head(0,0,0),tail(0,0,1),roll(0),euler_rotation(0,0,-pi/2)
globalQ = pose_bone.matrix.to_quaternion().copy()
parentQ = pose_bone.bone.matrix.to_quaternion().copy()
diff = globalQ.rotation_difference(parentQ) # becomes euler (0,0,pi/2)