Unexpected image result

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C#

When converting an Image to 8bpp there are vertical lines in result.
The Code used is,

static Bitmap To8bpp(Bitmap original)
        {
            int width = original.Width;
            int height = original.Height;
            Bitmap result = new Bitmap(width, height);
            for (int x = 0; x < width; x++)
            {
                for (int y = 0; y < height; y++)
                {
                    Color color = original.GetPixel(x, y);
                    result.SetPixel(x, y, color);
                }
            }
            Rectangle rectangle = new Rectangle(0, 0, width, height);
            PixelFormat pixelFormat = PixelFormat.Format8bppIndexed;
            BitmapData bitmapData = result.LockBits(rectangle, ImageLockMode.ReadOnly, result.PixelFormat);
            Bitmap bitmap = new Bitmap(width * 4, height, bitmapData.Stride, pixelFormat, bitmapData.Scan0);
            result.UnlockBits(bitmapData);
            return bitmap;
        }

What I have tried:

For whatever reason there are vertical lines in the resulting image. I have tried googling the reason for this with no success in explaining why this is occurring. If anyone understands what's happening here or knows of a good link that could better explain this would be great. Any help in this matter is highly appreciated.