Oki, from the point of view of mathematical you are wright. How you measure the gap? who has those measurement tools? Is prove already that the majority, I do not say all; when the "S" is concave is near. Is not me who state this it is the community.
Me if I have the coin I can measure. But even there the question is how much must be? No one know. If the third digit after point differed? it is far or near?.
So what was observe till now and the studies is shore are not in cement, it is say that the inside "S" upper right is concave in near and strait on far.
If you find better way please just post. Anyone will be glad to know and adapt.