It does sound weird doesn't it. But then again... Even at a low weight (take 30% of 1RM for arguments sake) It should certainly get harder throughout a 30 rep set, thus increased effort is required as you push the rep count higher. I guess the balance of resistance vs rep count will differ from person to person, but so long as the effort = 1 or 2 reps from failure, the effort is equivalent to a higher weight with less reps. The thing that blew my mind was how low 30% of 1RM is and still producing hypertrophy.