WPF Mouse Position and Visible dot not the same place

private void MainGrid_MouseDown(object sender, MouseButtonEventArgs e)
        {
            // Get the mouse click position relative to the mainGrid
            Point clickPosition = e.GetPosition(mainGrid);

            // Show a dot at the click position
            ShowDot(dotEnd, clickPosition);
        }
private void ShowDot(Ellipse dot, Point position)
{
    dot.Margin = new Thickness(position.X - dot.Width / 2, position.Y - dot.Height / 2, 0, 0);

    // Show the dot
    dot.Visibility = Visibility.Visible;
}