Public Quest loot rolls

[ALegendIsAfoot]

Hey, just recently started out and I've noticed a few things regarding public quests that just puts me off pretty hardcore -- The first time I hopped in, it was at Stage 2 already and the group invites I sent were rejected so I just kept at it. I did almost nothing, yet my loot roll pushed me above all three others who had been there from the start, and I got the golden bag.
Weird, but okay.
I didn't get credit for completing the PQ so I had to do it again, and I tried doing it solo. Pushed through Stage 1 and 2, then someone who was idling nearby woke up and attacked the boss on Stage 3, and their loot roll was boosted way above mine.
I've experienced both multiple times, doing all the work to get the lesser rewards and doing nothing while getting the best.

Why is the loot roll biased towards people who just wander in?