5.7 vs 4.8

I was just wondering this. The other day me and a co-worker raced. I have a 97 Sierra with a 5.7L vortec with True dual flowmaster and K&N intake and he has a 05 Silverado with a 4.8L vortec with straight pipes, both are rcsb also both 2wd, both automatic. We went from about a 15mph role, he was at my back bumper and punched it, I didn't punch it until he was about at my door. He pulled about a front bumper infront of me and we stayed there until about 75 then I started inching away from him. My truck is rated at 255hp and his truck is rated at 285hp shouldn't he have been able to out horsepower me and take off? Do I have more torque then the 4.8L and that is why I kept up and starting taking off? Or is my truck just underrated and closer to his hp?