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.
- 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).
- 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?
- 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.
- 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.
- 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?