You are right in thinking how you are but for a manufacturer to rate given parts at a required amount of hp would cause chaos when owners had their car's dynoed and the numbers were low. See 99 cobras. Also, when rpm increases in the drivetrain, more heat/friction occurs so more power is required to move said parts. So you may need 20hp to turn a 4l60e at 1500rpms but you need 50hp to move the same 4l60e at 5000rpms. So, a general percentage keeps the world sane
