why there is ear-shoulder connection?

[Idolphint]

I notice that the keypoint num is 18(if add background it canbe 19), the limb num is 19. there're two circle because of ear-shoulder connection.
the code has been specially deal that connection when drawing or grouping keypoints
why the connection should be here?
if the limb num and keypoint num has some relationship?

lokking forward to your reply!