WPF Drawing Paradox

Question

How is it possible, that 2 ellipses with the same Radius where not (visually) with the same Radius?

in the image bellow, Black and Red ellipses has the same RadiusX... but look on the picture!

<GeometryDrawing Brush="Red">
    <GeometryDrawing.Pen>
        <Pen Brush="Yellow" Thickness="1"/>
    </GeometryDrawing.Pen>
    <GeometryDrawing.Geometry>
        <GeometryGroup>
            <EllipseGeometry x:Name="MediumCircle" 
                             Center="0,0" RadiusX="4" RadiusY="4" />
        </GeometryGroup>
    </GeometryDrawing.Geometry>
</GeometryDrawing>
<GeometryDrawing Brush="Black">
    <GeometryDrawing.Geometry>
        <GeometryGroup>
            <EllipseGeometry x:Name="SmallCircle" 
                             Center="0,0" RadiusX="4" RadiusY="2"/>
        </GeometryGroup>
    </GeometryDrawing.Geometry>
</GeometryDrawing>

Answer 1

I think I found the reply, but not sure... The Thickness of the ellipse goes not totally around the ellipse. If we Set the Thickness to 4 we will see only 2 pixels of ellipse, if we set at 8, we will not see the red circle anymore...

Answer 2

One has a stroked border and the other doesn't. You'll notice the radius lines up with the center of the stroked border.

Answer 3

Looks like the thickness is to blame. Make them the same for both.