Choose a monospaced font for the editor.
The editor can only display monospaced. If the font is not monospaced, the editor defines the mono-spaced-grid width as the width of the widest char it finds.
The editor does not look at any attributes of the font. It always verifies a few chars to ensure the font is monospaced.
Among those chars are W and M. But also the @ sign, which in many fonts is even wider than M or W.