| | | this part of weight is not supported by bottom bit
v v v
+------------------------------+
| |
+-----------------------------------+
| |
+------------------------------+
^
|
now, wear is concentrated here; as you see this will
wear AT the edge of bottom piece and INSIDE the edge for top bit
It is empirically found that if that overhang is in 1/4 to 1/3 radius, very even grinding will result (= sphere). If strokes are longer, pieces will become hyperbolic (MORE wear at top-center & bottom-edge), if shoter, you'll end up with an oblate.
Bratislav