I think it’s because we tend to draw hand in a different fashion than drawing a head. I thought about it a lot when I started drawing hands again a few days ago. For example, I draw heads kind of layer by layer, I don’t do the right eye, then the mouth, then the ear... I kind of do everything at once. But when drawing hands, I used to forget about that and start drawing without knowing in advance where each part
https://youtu.be/UHElyke19_A?t=47