When is a semicircle not a semicircle?

If it all works as intended, four long curved pieces of Peco code 80 Setrack should form a semicircle.  The pieces that I have don't.

Tightly assembled, the ends of the curves don't line up.  The curve isn't quite of the right radius and when you do pull it to radius, the ends aren't quite parallel to the diameter of the circle.  This has made track laying more of a trial than it should be with Setrack.