does luck and creature's fame affect getting these items?

afaik luck...no, creatures fame yes, higher fame the more points they are worth

points? how does that work?

each creature is worth a point value based on their fame (no one knows how many points any creature is, except the devs) eventually as you kill more and more creatures your points add up (off screen for a lack of a better description) and when you hit the drop point value, a item appears in your backpack. There is also a message on your screen something about "for your valorous fight...blah, blah,blah. On your next drop, stop and go to a quiet area and open your journal to read the entire message.