It's a marketing ploy for people who don't know jack about hardware.
When it's phrased that way it makes the Xbox sound really powerful. 'Gee, why buy a PC they say right here that they'd have to tone it down to get it to work on a PC. The Xbox must be real powerful, hyuk, hyuk."
If they said "When this game is ported to the PC to make use of the Gef4 and 5 cards (the Xbox only has a Gef3), it'll really shine as only a game can on the PC!"
That wouldn't really inspire potential Xbox owners to go out and get one.
"Hrm, they say this thing'll look better on the PC, so wy should I buy an Xbox?"