First, take out the call to Refresh - it invalidates your form, and that causes another repaint!
Second, if you want the two sides the same, you need to draw two - or possibly four - curves, not just one.
A Curve is a Bezier (or more accurately a Cardinal Spline but for most applications you can ignore the difference), and they smooth out midpoints, so you won't get the same at the top, bottom, and right as you do at the start and end point on the left. If you draw the top and bottom separately, you may get what you wanted - I have no idea what you are actually expecting, so I can't be sure exactly what you need.
Bézier curve - Wikipedia
Cubic Hermite spline - Wikipedia