semi-circle oddity?

[gpholman]

Create a semi-circle (the method doesn't matter but I happened to use a straight line to slice a circle).

Using one of the semi-circles:

Set line width to 'none' and make the fill a light grey.

Clone it, make the clone black then enlarge clone to be 110%.

Put clone at back then select both objects and align top edges.

Why do you get a very thin line showing through the light grey semi-circle from the object beneath?

I've attached a .xar and .jpeg to illustrate.