Well, that's not really true.
Against these mobs, you get full XP if you deal more than 50% dmg, and I've tried running up to them dealing just the last little percentage and getting around 10% of the maximum XP.
I've used a warlock with a silly bot made in 2 minutes, that casts 3 shadowbolts at them, starting when they're around 60% HP, and I always get 800-900 XP, with 940 being maximum.
The fact that they're spread out doesn't matter when they have EXACTLY 3-4 seconds respawn time.
When doing it manually with my rogue, I need to wait 2 seconds for my stealth cooldown to fade before I attack them. That means I kill them in 4 seconds, they take 4 seconds to respawn, and I wait for them 2 seconds.
There, you satisfied?