A thought?
If each of the seam rays was just ONE thread too deep, that would have made the arc too small for the center.
The paper could have bunched up under it - maybe it shrank while you were working on it.
Anyway - if the arc is too small, can the center be "shaved down" a bit?
A tiny difference in radius makes a big difference in circumference:
Example: A four inch diameter circle (two inch radius) has a circumference of 12.57 inches
A circle with a diameter of 3.94 inches (1.97 inch radius) has a circumference of 12.19 inches. (This is less than 1/16 of an inch difference in diameters)
.38 inches - over 1/3 inch - which doesn't seem like much, but when working with unyielding fabrics like batiks, could make a difference.