If 6 members of a group have different targets they're going to monopolize the zone MORE then if every member of the group was looking for the same mobs. Because now they have to kill MORE mobs to satisfy everybody's quests.
example Everybody needs 5 each of mobs a, b, c, d and e. That's 25 mobs for the 6 people
If they each need 5 mobs, but split between a, b, c, d, e, f, g, h, i and j.. that is50 mobs for the same 6 people.
Edit: Unless you're saying the kills aren't group updates?