Strangeness
Much like the Troll Runestones, there are Boulders in all manner of strange places near the dam in Queensdale.
And someone else posted a screenshot of multiple spawns in the same spot of an animal/mob just like the Devourers. Lol.
I also remember a couple days ago running cross-country, from point A to vista B (lol), and was in the middle of some fields, far from any harpies, when one suddenly surprised me by attacking me! Next time through I saw it before it was in range, and attacked it, but still, there be harpies where there should be no harpies lol.
The troll runestones could just be someone (or some group of people) taking the runestone from the troll cave and running it to the hunting lodge for funsies, and then doing the same thing over and over.