Very good question! I started to investigate and after not finding any Swamp Troll around the area I suspected it could be a bug. And in fact, it is. If you select Chor in the Cyclopedia Map and zoom in north of Port Hope you'll see that in fact the Swamp Troll cave is also highlighted:
You can also see that by looking at the image mask the Client uses to highlight Chor (that's how I found it):
So the truth is that there aren't Swamp Trolls there, it's just a bug with the Cyclopedia map which also considers the entrance of the Swamp Troll cave north of Port Hope as part of Chor.