Every exercise has turned into a cheating exercise as most people are unable to leave their egos at the door.
The problem with pull ups is that you're not allowed warm up sets as you're stuck with your bodyweight as the be all end all. Eventually you're gonna get hurt.
With pulldowns, a mindset I've found helpful is to think about bringing your chest to the bar and that will cause you to bring the bar down and back with your arms serving strictly as levers.
Why are pulldowns harder with the same weight than pullups?
Anyone else think this?

What Adonis said. With pull-ups, most get that swinging motion going which adds momentum to the force applied.