Also, my personal experience seems to follow these rules as well. With the 3 previous campaigns, I was "late" getting to them. I was still finishing up the AO3 quests when I was lvl50, so I did not start the sewers until I was lvl52. I did not start Nuri until I was lvl58, and I did not start Fang until I was lvl61. The results are that I never got a single pink drop from any of those 3 campaigns. So at lvl63 I started Humania, and I've gotten 3 pink drops there so far. Again, this is a very small sample rate, but could indeed be the case that odds are slightly better when you are lower level. It does make sense that they could make the reward better for a more difficult fight.