I have a yellowed Super (my first one) and it was yellow before I got it from a non-smoking home. The problem lies in the chemical structure of the plastic used, specifically flame retardent plastics: http://retr0bright.wikispaces.com/Flame+Retardants
This is an excellent site as for correcting the issue on yellowed or browned consoles/computer. http://retr0bright.wikispaces.com/home Check the gallery for some SNES before and after photos too: http://retr0bright.wikispaces.com/The+Gallery