The way cairo maps CAIRO_HINT_STYLE_MEDIUM to Freetype settings is different from other libraries, which makes things look strange when you have some apps using Cairo for rendering and some using something else (like Xft).

The offending code is in cairo-ft-font.c:_cairo_ft_options_merge():

...
	switch (options->base.hint_style) {
	case CAIRO_HINT_STYLE_NONE:
	    load_flags |= FT_LOAD_NO_HINTING;
	    break;
	case CAIRO_HINT_STYLE_SLIGHT:
	    load_target = FT_LOAD_TARGET_LIGHT;
	    break;
	case CAIRO_HINT_STYLE_MEDIUM:
	    break;
	case CAIRO_HINT_STYLE_FULL:
	case CAIRO_HINT_STYLE_DEFAULT:
...

All other implementations I've found map FT_LOAD_TARGET_LIGHT to both "slight" and "medium". E.g. Xft:

	if (FC_HINT_NONE < hint_style && hint_style < FC_HINT_FULL)
	{
	    fi->load_flags |= FT_LOAD_TARGET_LIGHT;
	}

Pango:

      switch (hintstyle) {
      case FC_HINT_SLIGHT:
      case FC_HINT_MEDIUM:
	ft2font->load_flags |= FT_LOAD_TARGET_LIGHT;
	break;

One entertaining effect of this is gnome-appearance-properties which uses Xft to render the font examples, but then GTK+ uses Cairo for the actual font rendering, resulting in the examples showing something other than what you will get.