Digital BW, The Print

Wilhelm has rated the ultrachrome pigments (9600) on cotton rag papers 
at approx. 70-80+ years for full color prints and >100 years for B&W 
prints on the same papers with the same inks.  The B&W prints were made 
using the color inks, not black only.  I may have missed the fine 
print, but why the difference?