Some items drop for some characters, some items drop at some places. But without extensive research very little can be proven. So here are a few theories that might explain why you are not getting the items you want from the locations that are listed in this Wikia.

  1. The item may only drop for a specific class/faction. The reason why I suspect this is because I've had several Oracle's web pieces drop in heroic with my Cleric. And that's the ONLY set that has been dropping. But as soon as I started doing Heroics with my Death Knight only Blessed Cavalier's Armor has been dropping. Note also how the sets dropped are intended for opposite faction (but same class).
  2. Expedition items may be expidition specific. Maybe it's one set for each expidition? Or maybe the pieces of the same set are scattered over all the expeditions?
  3. The given location on world map items are mostly one drop, and then an assumption has been made. It might be too general, or already too specific.
  4. IF the unit improvements on expedition sets are random, it seems as if they are bound to be the same as one of the set bonuses.
  5. About Expedition artefact class/teir. Sometimes they are Legendary class, and their artefact level reflect this as well. Could they be from the higher levels? And as such, being the strongest form possible to drop?