I would assume that the engineers allowed for the c/c being under more weight ( up to around 19,500 lbs. ) Since more weight = more resistance, those trucks have lower power to save the trans. Just my theory. Put that same engine in a go-cart and you shouldn't experience any problems!
Yes, I agree. For driveline and engine duty cycle. Dodge did the same thing with their C&Cs. The C&Cs are rated 305/610 whereas the pickups are rated 350/650.