It's far more likely you will hit heroes who are closer to 100% hp rather than 0% hp across a game, because people like to stay near full hp and like to run away when low hp. Think about laning, old passive you hit someone at full hp, they run back and heal back up with tangoes. Entire laning phase you're mostly hitting someone above 50% hp because who stays in lane with <50% hp letting a Naix hit them? Same thing in team fights, you'll attack someone who is full hp and then will try to escape so if you have to switch to someone else the old passive was better. Sure, if you're hitting someone from full hp to dead then the new passive might be better, but that doesn't happen much across a game of Dota.