why does ControlPaint.DrawReversibleFrame seem to be drawn at an offset to the mouse location

bool isDrag = false;
Rectangle theRectangle;
Point startPoint;
private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
    isDrag = true;
    startPoint = ((Control)sender).PointToScreen(new Point(e.X, e.Y));
    theRectangle = new Rectangle(startPoint.X, startPoint.Y, 0, 0);
    }

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
    // If the mouse is being dragged,
    // undraw and redraw the rectangle as the mouse moves.
    if (isDrag)

    // Hide the previous rectangle by calling the
    // DrawReversibleFrame method with the same parameters.
        {
        ControlPaint.DrawReversibleFrame(theRectangle,
            this.BackColor, FrameStyle.Dashed);

        // Calculate the endpoint and dimensions for the new
        // rectangle, again using the PointToScreen method.
        Point endPoint = ((Control)sender).PointToScreen(new Point(e.X, e.Y));

        int width = endPoint.X - startPoint.X;
        int height = endPoint.Y - startPoint.Y;
        theRectangle = new Rectangle(startPoint.X,
            startPoint.Y, width, height);

        // Draw the new rectangle by calling DrawReversibleFrame
        // again.
        ControlPaint.DrawReversibleFrame(theRectangle,
            this.BackColor, FrameStyle.Dashed);
        }
    }

private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
    {
    isDrag = false;
    }