Assume that you turn up to a PUG one evening which downs the first few bosses in a raid. The next day, the raid leader continues with a different bunch of people and clears the rest. Do you feel that it was a joint achievement – we cleared raid instance X?
How about if it was a guild run, and you were on reserve the second night?
I think most people would feel a sense of ownership for the second achievement but not the first, even though they might have put in exactly the same amount of time personally.