Hmm.. an interesting topic to spend some time on. I'll throw in my own theorycraft and others can agree/disagree/ignore as they like. It's pure speculation.
First, observations.
I've noticed that if i take a 60+ toon to kill gargoyles in Xaviera the likelyhood of the Shimmering Eye dropping is fairly low. Add in a luck potion and the rate you'll see those Eyes goes WAY up. However, you never ever see 2 of them at the same time.
I've noticed on boss mobs, that there seems to be a set pattern that any given kill will follow. Like 1 armor/weapon out of set [item1, item2, item3] and 1 armor/weapon out of set [item4, item5, item6]. For example, I've solo'd the 1st KBN easy boss a gazillion times, and the pattern looks something like this:
- 1x. Tier 3 or tier 4 rune out of the basic set.
- 1x or 2x blue armor/weapon with "Rare" prefix in the name.
- 1x purple armor of [chain boots, plate shoulder, cloth shoulder, leather hat]
- 100x or 0x Power of Soul (blue throwing weapon)
- 100x or 0x One-Kill-A-Day (purple throwing weapon)
- maybe other trash drops that I don't pay attention
Now if I were a CompSci undergrad trying to code this in a lazy fashion, I might do it something like this (pseudocode):
a=getRandom(0,1)
if a between 0 and 0.1: include Harm III rune in loot
if a between 0.1 and 0.2: include Vigor III rune in loot
... etc, until all 100% possibilities covered and changing the break points if not 10 total runes possible
b=getRandom(0,1)
if b between 0 and 0.1: include Rare Balanced Cloth Armor in loot
if b between 0.1 and 0.2: include Rare Scourge Leather Armor in loot
... etc, until all 100% possibilities covered and changing the break points if not 10 total blue armors possible
c=getRandom(0,1)
if c between 0 and 0.1: include Rare Scourge Hammer in loot
if c between 0.1 and 0.2: include Rare Battlefront Dagger in loot
... etc, until
80% possibilities covered and changing the break points if not 8 total blue weapons possible.
d=getRandom(0,1)
if d between 0 and 0.25: include purple chain boots in loot
if d between 0.25 and 0.5: include purple plate shoulders in loot
if d between 0.5 and 0.75: include purple cloth shoulders in loot
if d between 0.75 and 1: include purple leather hat in loot
e=getRandom(0,1)
if e between 0 and 0.2: include 100x Power of Soul in loot
f=getRandom(0,1)
if f between 0 and 0.2: include 100x One-Kill-A-Day in loot
...
Do you see the numbers there that represent break points between getting one item or getting something else? Those probably would not be coded so simply as constants - they would come from multiple other things added/subtracted/multiplied with each other. That is how the level difference would come in, as 1 part of that. The luck potions/buffs/etc could effect some part of that as well.
The last paragraph couldn't be applied to all of the example however: The purple armor line for example totals to 100% and all items of same tier, so nothing to change there. The throwing weapon lines could have luck potion & level difference effecting whether it's 0-0.2=give loot vs. 0-0.3=give loot. Same for the "c" line being 80% vs. 90% chance of blue item.
This scheme would mean that you never get an item with luck pots/buffs that you couldn't get without them. However, the likelyhood of NOT getting anything in a given loot slot goes down.