Why the left side and right side is similar?

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C#6.0

private void Form1_Paint(object sender, PaintEventArgs e)
{
for (int m = 0; m < 101; m++)
{
Pen greenPen = new Pen(Color.Green, 3);
Point point1 = new Point(400 - m / 2, 200);
Point point2 = new Point(500, 100);
Point point3 = new Point(600 + m / 2, 200);
Point point4 = new Point(500, 300);
Point point5 = new Point(400 - m / 2, 200);

Point[] curvePoints = { point1, point2, point3, point4, point5 };

e.Graphics.DrawCurve(greenPen, curvePoints);
this.Refresh();
}
}

What I have tried:

After compile, how to get both sides similar in the figure.