I've approximated your vertical tail in this image -- but instead of a shaded view, I've used the Hidden view...
The blue lines that run spanwise and chordwise are in U,V coordinates. Those are the parametric coordinates that define the surface of the wing.
The SubSurfaces (including the control surface) are drawn in U,V space -- not X,Y,Z space. Try treating the blue grid as graph paper and plot out a 'straight' line in U,V space -- you will find that it is not a straight line in XYZ space.
So, for control surfaces, I try to do a little better. I calculate two intermediate points on each side and then draw three straight line (in UV) segments. Usually, this does a pretty good job. However, in situations like this where the UV coordinates are particularly warped (with respect to XYZ), the boundaries will appear curved.
Rob