It seems like some parts of sets are vastly more rare than the others. For example yesterday I was farming BC1 with a group and nobody got jewelry parts for Jailbreaker set, nor weapons (only I got 1 bow) we did like 5 runs, so that's 20 pieces of loot per boss. Same with other sets in the dungeon, we managed to get full attire but not jewelry and weapons, and we had tons of duplicates.
Another example is City of ash 1 burning spellweave inferno staff. I made countless runs and nobody in my party has ever got the staff.
Finally Mother's Sorrow Inferno Staff. It's really rare and thus costs a lot on the market, however I quite easily get Ice and Shock staves from the same sets.
Should it be that way or are those drop rates manipulated so they are rare when in fact they should be equal with other drops?