Hi Jason, the shapes are just not totally coincident.

This piece here is an exact circle which you can tell since it reports as having a radius value in the properties panel:




Note that this other piece does not report having a radius value, that means it is not an exact circular arc and is instead a generic spline curve that happens to be shaped pretty close to circular:




From the look of it I would guess that your non-circular one started out as a circle but then had its control points edited? When you edit a circle's control points it is replaced by a generic spline curve that will no longer be perfectly circular. It will deviate from a circle by some small amount. So you don't want to do that, instead you generally want to keep the circles perfect with no control point manipulation on them and make tangent lines coming off from those rather than trying to mush the circle to the line.

- Michael