The reason that X popped up is because it's a capital X rendered in Apple Garamond, which was the main font Apple used for marketing, packaging, manuals etc. from System 7 to late 2005(ish).

See: http://www.fontriver.com/i/maps/apple_garamond_map.png

The original OS X 'X' is just an Aqua glass rendering of the same letter in Apple Garamond.