Not sure why this happens but it does with many sets. I cannot remember the set names but I used to look for these to buy low and resell. For example a helm piece of a common set was hardly EVER in traders but all the other pieces were numerous. The commons ones sold for 8k or so while the helms sold for 100k as if you looked at all the main traders they just were never there hardly. People who didn't know they were rare just listed them low at 8 k or so. I'm assuming it still happens all the time. As stated it was happening with several sets and most likely more back then.

The same with the Psychic Ambrosia pieces, you get them doing daily crafting writs and one of the pieces I never have while I have multiples and multiples of the others. None of them sell for a bunch but it's really noticeable.

I don't trade any longer but I don't think it is RNG, it's a programming bug maybe.