The username font system is a smidgen tedious, but it's simple if you know how it works.
The list above (thanks by the way) is a list of fonts that you can change your username font to, and everyone using the Windows operating system will see them (If they haven't deleted any fonts from their font folder).
If however you download a font (Like pV Irish has done) you can change your username font to that font, however only people that have that font installed on their computer will be able to see the font like that. If you don't have the font installed, it will appear as the default font on the forums.
Hope this clears things up
//AC