You can't compare roller cam specs and flat tappet specs directly. Roller lifters contact the cam at one narrow spot. Flat tappet specs are affected by the whole lifter face (3/4" or so) contacting the lobe as the lobe rotates. The wider the lifter face, the longer the duration. Peak lift is the same with both styles. The centerline of the flat tappet lifter is offset from the centerline of the cam lobe..this allows the lifter to spin in it's bore and wear more evenly.