it's coming from a misunderstanding of chance. saying that something has a 10% chance of occuring does not mean 1 out of 10 times you will get the desired result. i tried explaining this when explaining how random works but it's apparently not yet understood. chance is never a guarantee of something, flip a coin and theres a 50% chance it will land heads up. does this mean in 2 flips it will end up heads up once? no it does not, its just as possible that it would heads down both times. flip the coin 4 times, will it land up 2 of the times? maybe, but it could also land heads down every time, or 3 of the times. flip it 10,000 times, it could appear with a 46% appearance of heads. it could also appear with 0 heads. this is very unlikely but completely possible. go on for infinity coin flips. it might never actually land heads up. will it? probably not. but its completely possible it might.
just like resetting the merchant as many times as you like might never avail you the affix you wish to gain. that doesnt mean it can't. it just means you happen to have gotten really unlucky. chance is never a guarantee. you can still get the necklace at the level cap. the chance of you getting it is much lower as already discussed here as to why. which means it may take 1 try to find it, it may take 50 tries, it may take 200 tries. it may never even happen for you. but that doesnt mean it can't happen.
if i havent made the point clear by now i daresay i never will. but i can only hope this casts some illumination on the subject for you.
You don't seem to be understanding that I understand what you're stating and I understand random and chance. Believe me, I do. But I also understand impossibility. Is that something you understand? If it's impossible for something to happen it will not happen. Simple, no? In any case, I did extensive testing which has lead me to my current theory.
I theorize that the Necklaces of Disease or Poison resistance override each other. Let me paint a clearer picture.
* Necklace of Poison Resistance begins showing up at level 5.
* Necklace of Disease Resistance begins showing up at level 10.
* Necklace of Poison Immunity begins showing up at level 15.
* Necklace of Disease Immunity begins showing up at level 20.
While Necklace of Disease Resistance can show up, you will never see a Necklace of Poison Resistance. While a Necklace of Poison Immunity can show up, you will never see a Necklace of Disease Resistance. While a Necklace of Disease Immunity can show up, you will never see a Necklace of Poison Immunity.
It appears as though they're incorrectly overriding each other instead of just overriding their corresponding fields (I.e. Necklace of Disease Immunity overriding Necklace of Disease Resistance and Necklace of Poison Immunity overriding Necklace of Poison Resistance). I imagine this is a bug and not intended behavior, but knowing Bethesda I'm not quite sure.