I think a lot of this has to do with the way loot chances go. There is some declining % chance depending on where the item is located in the loot table. For example, lets take the classic Lady M example:
When killed, this NPC drops :
# With a probability of 100% (multiplier : 2):
1. Mithril Boots, Armor
2. Mithril Gauntlets, Armor
3. Mithril Helm, Armor
4. Orb of the Crimson Bull, Gems
5. Ebony Headband, Armor
I think we'll agree (those who have camped her) that there is not the same chance for an orb or headband as there is for the mithril items. These examples don't only pertain to PoFire, but are in every zone which has mobs with many drops. What is the rhyme or reason to which is more rare? I have no clue as some, such as Lady M, have an obvious order, but then others seem to be reversed such as Pyronis and his sphere.