It'd really be pretty unreasonable for ??? items' nature to be determined at the time of appraisal rather than at the time of creation. Both from a programming standpoint and from an attempt to apply real-world logic to game logic.
The latter argument is, of course, that it's some specific thing whether you've identified it or not. Only difference is that after identifying it, you know what it is, but it's still the same thing while it has its layer of dust on it or whatever.
As for the former, it's more efficient to make the chest determine the item from a small list specific to the source (i.e. ??? Sword chest from Assault A gives 40% sword 1, 35% sword 2, 20% sword 3, 5% sword 4, while ??? Sword chest from Assault B gives 50% sword 2, 40% sword 5, 10% sword 6). And then just stick the "what this item really is" into the ??? item as a hidden variable (like WS points on latent weapons, or kill count on that DRK quest sword). Then all the appraiser NPC's code would be would be "take item, see what its hidden variable says, and drop that into the treasure pool". Easy appraiser code, relatively easy item generation code.
The alternative would be to have the chest drop some item with a hidden variable saying where it came from (Assault A or Assault B, above), but then the appraiser NPC's code would be this big ugly mess that would have to handle every item source and calculate out all the odds at that time. Seeing as you'd have to put a hidden variable onto the item *anyway* (because it's confirmed that ??? items from specific sources can only identify into specific things based on the source), it'd be far easier to just make that hidden variable be what the thing actually is, and save yourself a step. Not to mention make your code more readable.
This is, of course, no guarantee that it actually *is* done logically (both in terms of real-world logic, and programming efficiency). It could very well be the other way around.
Personally I just think the Nashmau NPC is a little cheaper because it's all the way in Nashmau, and the Al Zahbi NPC is a little cheaper because he can be captured, whereas the Whitegate NPC is convenient and always present, and therefore has a higher fee to match. That strikes me as being more in line with the way S-E prices things.